Added slices to CoordinatesMap

.gitignore

@@ -14,3 +14,6 @@ doc/modules
 tmp/
 *.xtcindex
 .spyproject/
+.traj.xtc_offsets.lock
+.traj.xtc_offsets.npz
+*.npy

mdevaluate/coordinates.py

@@ -362,6 +362,7 @@ class CoordinatesMap:
         self.frames = self.coordinates.frames
         self.atom_subset = self.coordinates.atom_subset
         self.function = function
+        self._slice = slice(None)
         if isinstance(function, partial):
             self._description = self.function.func.__name__
         else: