ENH: support reading box vectors from TPR files

[tylerjereddy]

As noted at #5374 (comment):

On the latest version of that feature branch (and presumably on develop),

import MDAnalysis as mda
from MDAnalysisTests.datafiles import TPR, XTC

u = mda.Universe(TPR, XTC)
print(u.dimensions)
u = mda.Universe(TPR)
print(u.dimensions)
u = mda.Universe(TPR, TPR)
print(u.dimensions)

produces:

[80.017006 80.017006 80.017006 60.       60.       90.      ]
None
None

I need to keep requests related to TPR parsing scoped within reason, so opening this issue.