doc added; reading of isochronal bds

doc/source/user_guide/fit.rst

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+.. _user_guide.fit:
+
+============
+Fitting data
+============
+
+.. image:: ../_static/fit_dialog.png
+    :scale: 80%
+    :align: center
+
+The picture gives an example of dialog to setup and start fits.
+First, there is the possibiity to fit different functions, called models to differentiate from the functions inside each
+model, to different data simultaneously.
+In the given example, two models were would be used during fit:
+test1 and test3 use the default model, in this case model a,
+while test2 uses model b.
+
+In the middle column, the functions of model a are currently given in a tree structure:
+It is comprised of three functions: a constant, a sine curve, which is a child of the constant, and free diffusion.
+Functions can be dragged by mouse to each position, including a child position, and it is possible to switch between the
+four basic arithmetic operations (+, -, \*, /) by striking the corresponding key.
+Function at the same level, e.g., constant and free diffusion, are evaluated in the given order.
+Children of a function take precedence over the following functions.
+This means for the given example, that model a is something like
+
+.. math::
+    f(x) = [\text{constant} / \text{sine}] + \text{diffusion}
+
+It is possible to skip functions by de-selecting it.
+This choice alse affects children, even if they are selected.
+
+The right column gives access to the parameter of selected function, in this case of the free diffusion function.
+Here, the parameter :math:`M_0` is linked to the parameter :math:`C` of another constant function of model b
+(identifiable by the number behind the name).
+Linkage links not only parameter between function but all settings, e.g., if :math:`C` is fixed, :math:`M_0` is fixed.
+Some parameters like gradient :math:`g` are predefined as fixed parameters and are lacking any options besides their value.
+Upper and lower bounds may be given.
+If one or two of the bounds are not given, this parameter is unbounded on the respective side.
+
+If a parameter has one entry, this value is initial parameter for all data sets.
+If the number of entries is greater than the number of data sets, the first :math:`n` values are used.
+Here, test1 uses 1 as parameter for :math:`t_{ev}` and test3 uses 3.
+If the number of entries is less than the number of data sets, the last value will be used for each additional set.

doc/source/user_guide/index.rst

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+==========
+User Guide
+==========
+
+
+.. toctree::
+   :maxdepth: 2
+
+   read
+   fit
+   shift_scale

doc/source/user_guide/read.rst

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+.. _user_guide.read:
+
+*************
+Reading files
+*************
+
+Supported filetypes are
+
+   * Text files,
+   * DAMARIS HDF5 files,
+   * Grace images.
+   * HP alpha-analyzer EPS files
+   * NTNMR .tnt files
+
+DAMARIS HDF files
+=================
+
+After scanning the selected file the program shows a list of the available data.
+

doc/source/user_guide/shift_scale.rst

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+.. _usage.shift_scale:
+
+Moving and scaling
+^^^^^^^^^^^^^^^^^^
+
+Values are always used and displayed in scientific notation *x.yyyyez* where x is a signed single digit larger than 0.
+The relative stepsize (by default 0.0001) increases or decreases is always of the same magnitude as the current value, e.g.,
+1.2345e0 changes by 0.0001, but 1.2345e20 changes by 0.0001e20.
+If the step results in a new order of magnitude the value will be adjusted accordingly, i.e., a step from 9.9999e1 to 10.0000e1 will become 1.0000e2.
+
+.. note::
+    This behaviour leads almost always to values different from zero when using the arrow buttons. Please enter it directly to get a value of zero.
+
+Boxes :guilabel:`log x` and :guilabel:`log y` change the respective axis from a linear to logarithmic scaling and vice versa.
+
+Every selected set will be recalculated according to :math:`new = scale \cdot old + offset`.