ImageSource.save() does not write Relion >=3.1 starfiles

[j-c-c]

While working on #1302 I checked to see if we can save an ASPIRE source and load it in Relion >=3.1. This can not be accomplished without alteration of our saved starfile. The main problem is that we do not save starfiles with an opticsGroup column, which is the Relion >=3.1 convention. We are able to read in this convention with a RelionSource or CoordinateSource, but we reorganize the metadata and do not reconstitute it when saving.

Relion does offer a relion_convert_star method which attempts to convert from the old convention to the new by adding an optics group and attempting to convert from pixel-valued offsets to angstrom-valued offsets. This conversion is unsuccessful in that it does not correctly convert the offsets (for reasons I have not yet figured out) and does not move the _rlnImagePixelSize into the optics group (which causes an Error when loading).

A little more investigation is needed, but ultimately I think we need to update our save method.

Any possible changes from #745 could be completed with this update.