Prevent creation of groups of floats

[fingolfin]

Fixes part of issue #823.

[codecov]

Codecov Report

Merging #2315 into master will increase coverage by 0.15%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2315      +/-   ##
==========================================
+ Coverage   72.17%   72.33%   +0.15%     
==========================================
  Files         479      479              
  Lines      250960   250963       +3     
==========================================
+ Hits       181140   181537     +397     
+ Misses      69820    69426     -394

Impacted Files	Coverage Δ
lib/float.gi	31.86% <100%> (+0.37%)	⬆️
hpcgap/lib/hpc/queue.g	66.4% <0%> (-3.2%)	⬇️
src/objset.c	81.27% <0%> (-0.23%)	⬇️
src/gap.c	80.29% <0%> (-0.09%)	⬇️
lib/grp.gi	85.47% <0%> (+0.07%)	⬆️
hpcgap/lib/hpc/stdtasks.g	38.87% <0%> (+0.25%)	⬆️
lib/mat8bit.gi	67.32% <0%> (+0.99%)	⬆️
src/macfloat.c	80.48% <0%> (+1.21%)	⬆️
src/vec8bit.c	83.25% <0%> (+4.7%)	⬆️
src/vecgf2.c	71.78% <0%> (+7.78%)	⬆️
... and 1 more