Implement biomass sampling experiments for metabolic networks in C++

[TolisChal]

This PR implements C++ tests for sampling from the flux space of 3 metabolic networks:

1. e_coli
2. iAT_PTL_636
3. Recon1
   with respect to the exponential distribution induced by the biomass objective function of each model.

The PR adds the e_coli polytope and its objective function in a new folder metabolic_full_dim in folder tests.
The tests for 2. and 3. are commented out from CMakeLists.txt because the .ine files are too big.

One could run the test for e_coli by running:

logconcave_sampling_test -tc=e_coli_biomass

[vissarion]

Thanks for this PR!
There is an issue in CI tests for e_coli_biomass, to my understanding it shouldn't take that long to execute.

[vissarion]

BTW, instead of commenting out the new tests, you could create a new test file that is not tested in CI but a user can test it on demand locally (without editing/uncommenting code in the test files).

[papachristoumarios]

iAT_PTL_636_biomass -> iAT_PLT_636_biomass.
@TolisChal Can you also create a test that calls the three tests? Since on gaussian sampling we have it like this, I think it should be a good idea to keep it this way for the exponential case as well.