Add IsCentral methods where second argument is an element

[fingolfin]

The documentation for IsCentral already claims these methods exist, but they never did. Add them in a systematic fashion.

Resolves #1795

[markuspf]

We should probably add some tests for them as well.

[fingolfin]

Yeah but I was too tired to add those now, too. I'll revisit this PR later (but in the meantime, didn't want to loose these changes in my stack)

[codecov]

Codecov Report

Merging #2185 into master will increase coverage by <.01%.
The diff coverage is 25%.

@@            Coverage Diff             @@
##           master    #2185      +/-   ##
==========================================
+ Coverage   69.72%   69.72%   +<.01%     
==========================================
  Files         482      482              
  Lines      254483   254483              
==========================================
+ Hits       177435   177443       +8     
+ Misses      77048    77040       -8

Impacted Files	Coverage Δ
lib/ring.gi	51.93% <ø> (ø)	⬆️
lib/algebra.gi	72.45% <ø> (ø)	⬆️
lib/grp.gi	84.42% <ø> (-0.06%)	⬇️
lib/magma.gi	50.29% <ø> (ø)	⬆️
lib/magma.gd	79.31% <25%> (+17.4%)	⬆️
src/hpc/threadapi.c	36.9% <0%> (-0.1%)	⬇️
src/hpc/traverse.c	95.47% <0%> (+0.02%)	⬆️
src/opers.c	81.47% <0%> (+0.05%)	⬆️
src/stats.c	85.17% <0%> (+0.13%)	⬆️
hpcgap/lib/hpc/stdtasks.g	38.87% <0%> (+0.25%)	⬆️
... and 2 more

[fingolfin]

I added some tests. Let's see how Travis and codecov like them.

[markuspf]

(Subject to the tests passing, etc..)