Allow passing Phase instead of Symmetry

[hakonanes]

As @CSSFrancis pointed out in #478 (comment), it may sometimes be simpler to pass a Phase with a point group set, rather than extracting the corresponding Symmetry, to functions like the sampling functions. Since a Phase only has one Symmetry, this should not be a problem.

I'm for this. What do other people think?

[CSSFrancis]

@hakonanes Seems simple enough and would be a nice addition

[harripj]

I think this would be a convenient addition to sampling.get_sample_reduced_fundamental. Would this also be applicable for other parts of the API?

[hakonanes]

That was my intention, yes,wherever it makes sense.

[hakonanes]

I'm not sure of the best implementation. Should we allow passing a Phase to functions with a point_group parameter? What about to the space_group parameter? Or should we replace these parameters with one which can be all these types? I haven't thought this through, but realize it requires some thought 🤔.

[harripj]

Would it be enough to improve the docstrings and examples with an example extracting the point_group from a Phase object?