DOC: make some of the docstrings conform with NumPy style

Merged Vasudevan, Rama K requested to merge github/fork/magnunor/DOC_docstring_fixes into master

Created by: magnunor

I went through the doc-strings and changed them so they follow the NumPy style, which is also the same style which is used in SciPy, matplotlib and HyperSpy.

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