Mcmc unitary tests

[MerlinKeller]

I added three unitary tests to the MCMC classes to the existing Python test files:

• 2D normal target test to Gibbs class in t_Gibbs_std.py
• test for IMH based on Beta-binomial model t_IndependentMetropolisHastings_std.py
• test for RVMH based on Beta-binomial model t_RandomVectorMetropolisHastings_std.py

[mbaudin47]

Thank you for this significant improvement! Only minor changes will greatly improve these unit tests.

[josephmure]

I don't understand the error which occurs:

/home/devel/project/lib/test/t_RandomVectorMetropolisHastings_std.cxx:72:30: error: parentheses were disambiguated as a function declaration [-Werror=vexing-parse]
   72 |     RandomVector randomVector(Normal());
      |                              ^~~~~~~~~~
/home/devel/project/lib/test/t_RandomVectorMetropolisHastings_std.cxx:72:30: note: replace parentheses with braces to declare a variable
   72 |     RandomVector randomVector(Normal());
      |                              ^~~~~~~~~~
      |                              -
      |                              {        -
      |                                       }
/home/devel/project/lib/test/t_RandomVectorMetropolisHastings_std.cxx:73:63: error: no matching function for call to ‘OT::RandomVectorMetropolisHastings::RandomVectorMetropolisHastings(OT::RandomVector (&)(OT::Normal (*)()), <brace-enclosed initializer list>)’
   73 |     RandomVectorMetropolisHastings sampler(randomVector, {0.0});

[MerlinKeller]

@jschueller : this PR can now be merged into the 1.23 branch imo ! After the reviewer's approval...

[mbaudin47]

@jschueller : This PR is ready for merging. Can you please review or merge?