add FitResult fit overlays and close leaked HDF5 files after experiment stop

FitResult is a new Drawable in damaris.data with x-values independent of
the measured points, so fits can be sampled on a dense grid. It is stored
as its own data pool entry (e.g. "measurement/fit"), written to and read
from the HDF5 data pool (damaris_type "FitResult", params as attributes),
and drawn by the GUI as overlay on its measurement, including a fit
parameter text box. Fits get no entry in the display tree, so measurement
rows stay directly selectable. Result scripts create them via
FitResult(x, y, title, source, params).

Additionally, HDF5 files that experiment or result scripts left open
(e.g. after a script failure) are closed when the experiment stops.
pytables keeps such handles registered process-wide, which blocked
reopening the dump file and the scripts' own files on the next run.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

tests/res_test.py

@@ -1,6 +1,6 @@
 import tables
 import numpy
-
+# from damaris import FitResult
 r = MeasurementResult("test")
 
 def result():
@@ -16,6 +16,19 @@ def result():
         ts.write_to_hdf(h5, "/", f"adc_{num}", f"adc_{num}")
         data["fft"] = (ts+0).fft()
         r[num] = ts.y[0][0]
+        if num > 4:
+            x = r.get_xdata()
+            y = r.get_ydata()
+            coeffs = numpy.polyfit(x,  y, 2)
+            x_line = numpy.linspace(x.min(), x.max(), 100)
+            y_line = numpy.polyval(coeffs, x_line)
+            
+            fit = FitResult(x=x_line,
+                    y=y_line,
+                    title="Magnetization(tau) fit",
+                    source="test",
+                    params={"a": coeffs[0], "b": coeffs[1], "c": coeffs[2]})
+            data["test/fit"] = fit
         data["test"] = r
         accu+=ts
         print(accu)