Xml2star conversion particle path bug

[McHaillet]

Pytom writes particle annotions in .xml format, but for compatibility with Relion and Warp it needs to be converted to a .star file format. Pytom already supported conversion to a simplistic pickpostion format that could be read by warp for subtomogram reconstruction. I now added support to allow back and forth conversion with star files that refer to subtomograms and ctf volumes that Relion needs for subtomogram averaging and classification.

• this prepares pytom for some subtomogram averaging update I wanted to add
• and closes #126

I updated unittests to make sure the old behaviour is consistent, and the back and forth conversion with the updated format is consistent (no information loss).

[McHaillet]

and I would very much also like unittest for the newly added fail paths (so the code should fail with the expected error if it is asking with tomo data without the new keywords)

Good point, will do!

[McHaillet]

I updated as requested:

• added tests that check for error raising by pl2star for missing spherical_aberration or voltage

And also changed the following:

• removed ERROR text from pl2star because exception should already be informative enough. It also made the unittest confusing.
• updated the IOTest to not read an explicit relion star header, star file reading/writing should be able to read any starfile and write any numpy structured array to a star