Use NrRows and NrCols and mat[i,j] in more places

[fingolfin]

No description provided.

[codecov]

Codecov Report

Merging #40 (c21f167) into master (115b07d) will decrease coverage by 0.02%.
Report is 3 commits behind head on master.
The diff coverage is 20.93%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
- Coverage   47.62%   47.60%   -0.02%     
==========================================
  Files          34       34              
  Lines       17927    17910      -17     
==========================================
- Hits         8537     8526      -11     
+ Misses       9390     9384       -6     

Files Changed	Coverage Δ
lib/diagram.gi	21.09% <0.00%> (ø)
lib/gpolygons.gi	24.51% <0.00%> (ø)
lib/morphisms.gi	21.10% <0.00%> (ø)
lib/orbits-stabilisers.gi	42.94% <0.00%> (ø)
lib/polarspace.gi	61.60% <0.00%> (ø)
lib/correlations.gi	45.09% <9.09%> (-0.05%)	⬇️
lib/group.gi	63.34% <42.22%> (-0.02%)	⬇️
lib/projectivespace.gi	55.93% <46.15%> (ø)

... and 2 files with indirect coverage changes