Add doublecoset benchmark

[olexandr-konovalov]

The test from tst/testextra/doublecoset.tst, added in #2741 by @hulpke, requires more memory than in the default make teststandard setting, so it is moved to benchmarks, with two more shorter tests added.

[hulpke]

I approve as I do not like the idea of blocking merging from system authors until a PR has been approved.
However the two extra examples used do not contribute meaningfully to a test of the new features.

[hulpke]

This isn't really a benchmark for double cosets, as the index is small enough to even calculate orbits on cosets. (In fact the whole calculation on a group of order barely 600k is a very easy case)
The issue that the new code addresses is with subgroups of large index that do not have satisfactory intermediate subgroups (which is a given if they are maximal), so that a Leiterspiel with up-step is required.

One could try to have larger examples without Leiterspiel (just requiring to find intermediate subgroups), but as long as indices are not in the millions this is all childs play.

[olexandr-konovalov]

@hulpke Thanks - I will adjust the names of the tests before merging this.

The reason to have three tests is that all other benchmarks also have three tests, and it's easy to have a common script in Jenkins which is parametrised by the name of the directory:

cd benchmark/$BENCHMARK
../../bin/gap.sh -o 1g test1.g
../../bin/gap.sh -o 1g test2.g
../../bin/gap.sh -o 2g test3.g 

and then what I see is

The idea is that the three tests are of increasing difficulty. If you could contribute two examples, one faster than Co3, and another either faster or slower, I will be happy to use them as more meaningful!

[codecov]

Codecov Report

Merging #2979 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2979      +/-   ##
==========================================
+ Coverage   85.21%   85.21%   +<.01%     
==========================================
  Files         110      110              
  Lines       56857    56857              
==========================================
+ Hits        48450    48453       +3     
+ Misses       8407     8404       -3

Impacted Files	Coverage Δ
src/iostream.c	63.49% <0%> (+1.14%)	⬆️

[olexandr-konovalov]

I've updated test files - now I'd rather merge this and changed examples in benchmarks later, to get it out of the way of make teststandard which fails in Travis because of the memory demands of tst/testextra/doublecoset.tst.