Upgrade to GSL v2.6

[giordano]

Just using the new binaries makes tests pass locally, let's see how it goes with the rest of CI.

I guess we may want to upgrade the wrappers too (see #102), so I'm marking this pull request as draft.

Fix #102.

[codecov-io]

Codecov Report

Merging #103 into master will decrease coverage by 0.04%.
The diff coverage is 1.55%.

@@            Coverage Diff            @@
##           master    #103      +/-   ##
=========================================
- Coverage    7.04%   6.99%   -0.05%     
=========================================
  Files         189     189              
  Lines        5282    5348      +66     
=========================================
+ Hits          372     374       +2     
- Misses       4910    4974      +64

Impacted Files	Coverage Δ
src/gen/direct_wrappers/gsl_integration_h.jl	6.81% <ø> (ø)	⬆️
...gen/direct_wrappers/gsl_permute_vector_double_h.jl	0% <ø> (ø)	⬆️
src/gen/direct_wrappers/gsl_filter_h.jl	0% <ø> (ø)	⬆️
src/gen/direct_wrappers/gsl_randist_h.jl	0% <ø> (ø)	⬆️
src/gen/gsl_types.jl	100% <ø> (ø)	⬆️
src/gen/direct_wrappers/gsl_vector_double_h.jl	10.86% <0%> (-0.25%)	⬇️
src/gen/direct_wrappers/gsl_vector_ushort_h.jl	0% <0%> (ø)	⬆️
src/gen/direct_wrappers/gsl_vector_ulong_h.jl	0% <0%> (ø)	⬆️
src/gen/direct_wrappers/gsl_vector_short_h.jl	0% <0%> (ø)	⬆️
src/gen/direct_wrappers/gsl_matrix_char_h.jl	0% <0%> (ø)	⬆️
... and 27 more

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[ludvigak]

Nice! I re-generated the wrappers from the 2.6 source, let's see if it passes...

I had to ignore the new gsl_bst* stuff, because it uses C unions, which I don't know an easy way of dealing with.