Introduce classes for ProjData indices

[KrisThielemans]

This PR introduces SegmentIndices, ViewgramIndices and SinogramIndices (and derives Bin from ViewgramIndices ) and add constructors and methods to use these, as opposed to listing indices separately. This will allow for more concise code, but also prepares for TOF (and energy/layers etc).

Using these new methods is left for the TOF branch.

DataSymmetriesForViewSegmentNumbers etc have not been modified yet.

Fixes #1263

[KrisThielemans]

temp zenodo problems. Also, Appveyor didn't run, as last commit was only for the release notes. I'll need to push something else.

[robbietuk]

I agree with the change and it looks good. I dont have time to fully review it. Are there any additional tests that should be added?

[KrisThielemans]

Are there any additional tests that should be added?

I guess so. However, the old functions are now mostly implemented in terms of the new ones, so we have the tests for free. There will be a few untested, but we will find out once we apply this to TOF.

I wanted to have this PR on 5.2, such that we can tell people to use the new interface, and it will work for the next STIR version as well (while the old functions won't work for TOF). Maybe I didn't make the latter very clear in the release notes....

[KrisThielemans]

@danieldeidda this should be ready now, aside from some tiny edits to the release notes.

[danieldeidda]

looks good to me