SymmetryGroup Parser

[j-c-c]

We are currently duplicating some symmetry_group parameter parsing logic around the code (Image.backproject(), ImageSource.symmetry_group). This just be part of a general parsing method, possibly just add the logic to SymmetryGroup.from_string()

Idea mentioned here: #1072 (review)

[j-c-c]

Closed with #1106.