Properly pluralize the ViewString for pregroup presentations

[wilfwilson]

This makes use of the Pluralize function that was introduced in gap-system/gap#3992, and will be included in GAP 4.12.

Obviously we can't require GAP 4.12 yet, because it doesn't exist, and even once it is released the use of Pluralize isn't in itself a good enough reason to require it.

So for now I have duplicated the relevant subset of the Pluralize function in this package. This can be removed once this package requires GAP 4.12 or later.

[codecov]

Codecov Report

Merging #34 into master will decrease coverage by 0.16%.
The diff coverage is 58.33%.

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
- Coverage   90.10%   89.93%   -0.17%     
==========================================
  Files          22       22              
  Lines        2364     2374      +10     
==========================================
+ Hits         2130     2135       +5     
- Misses        234      239       +5     

Impacted Files	Coverage Δ
gap/examples.gd	100.00% <ø> (ø)
gap/presentation.gd	100.00% <ø> (ø)
gap/walrus.gd	100.00% <ø> (ø)
gap/presentation.gi	91.28% <58.33%> (-1.99%)	⬇️