Ensure that GRO reader treats lack of unit cells consistently

[IAlibay]

Fixes #3305

Changes made in this Pull Request:

• Add warning on reading / writing GRO files with empty box
• Remove support for attempting to read GRO files with unit cells with neither 3 of 9 entries

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

[IAlibay]

@lilyminium @mnmelo @jbarnoud I know you have many views on this, please do review.

[codecov]

Codecov Report

Merging #3380 (0d1d234) into develop (6531904) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #3380      +/-   ##
===========================================
+ Coverage    93.62%   93.64%   +0.02%     
===========================================
  Files          176      176              
  Lines        22839    22839              
  Branches      3225     3225              
===========================================
+ Hits         21382    21387       +5     
+ Misses        1406     1401       -5     
  Partials        51       51              

Impacted Files	Coverage Δ
package/MDAnalysis/coordinates/GRO.py	96.64% <100.00%> (+2.85%)	⬆️
package/MDAnalysis/coordinates/PQR.py	95.23% <0.00%> (+0.91%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6531904...0d1d234. Read the comment docs.

[lilyminium]

My brain hasn't really woken up yet, but I found those negatives hard to parse. Maybe telling the user what boxes are allowed is easier?

[lilyminium]

LGTM thank you @IAlibay, I'll let you decide if you want to wait on further reviews or merge