GRO reader / writer, handle missing / incomplete unitcell data

[IAlibay]

Follow up from #3132

See comments: https://github.com/MDAnalysis/mdanalysis/pull/3132/files#r628893880, https://github.com/MDAnalysis/mdanalysis/pull/3132/files#r628897097, and https://github.com/MDAnalysis/mdanalysis/pull/3132/files#r628897541

Our default behaviour when reading / writing missing/incomplete unitcell data is both undocumented and probably inconsistent with gmx.

I would suggest here that we just be harsh, if there's missing/incomplete unitcell on reading or writing we should just throw an error and tell users what they should do to fix things (we can couple this with a transformation class that sets a unitcell that exactly accounts for the coordinates).