minor refinements to headers in SPR and YPR output

[iantaylor-NOAA]

Changes SSBpR to SPR and YpR to YPR in table headers and comments. This causes the terminology used in the output differ from variable names in the TPL, but it makes the output internally consistent and more consistent with previous versions of SS3 and common terminology, which will also simplify processing by r4ss. Suggesting these as a Pull Request to the in-process branch seemed easier than describing the changes in a comment on the existing PR #640.

The one change I was least confident in is SPR_virgin. It seems like it could be either SPR0 or SPR_virgin, but SPR0_virgin seems redundant.

Changes are only to comments in Report.sso so should not impact anything numeric.

@Rick-Methot-NOAA if you think any of these changes aren't good, feel free to close this PR or change things around.

[Rick-Methot-NOAA]

I will take a look. My intention with the extensive refactoring was to clarify that:
SPR = SSBpR(fished) / SSBpR(unfished)
but I may have missed some.
The original code often used SPR when the quantity was actually a SSBpR

[iantaylor-NOAA]

Of course, sorry for completely forgetting the difference between the Spawning Potential Ratio and the Spawning Potential per Recruit. In that case it's better to make a change and not worry about matching the older SS3 because we wouldn't want people comparing SPR to SSBpR and thinking they were the same thing.

Feel free to pick and choose any changes that seem userful, and/or just close this PR.

[Rick-Methot-NOAA]

capitalization in line 4795 is good change.
line 4797 with word "stored" can be safely deleted as it looks like a Rick testing line
other changes should not be made

[iantaylor-NOAA]

Sounds good. I just made those change directly in a commit 7d972b3 to the 191-spawner-recruit-rebased branch so will now close this PR.