DOC: improve IDIMethod documentation

[domengorjup]

Some IDI methods have lacking documentation.

For example, the LucasKanade.calculate_displacements docstring should give more information on the shape and other properties of the calculated displacements.

The documentation should be unified across other IDI methods as well, or perhaps clarified further in pyidi/idi_method.py.

[domengorjup]

For pyIDI.get_displacement: could the main properties (shape etc.) of the computed displacements be defined in this docstring, before appending the method-specific doumentation?