Endf get cov

[AitorBengoechea]

First step towards the direct extraction of covariance matrices in a single method.

_get_xs_cov: Extract the covariance matrix based on the information of mf=33 and mf=31. Tested with sandy.XsCov, since the original pipeline was based on this class.

_get_lpc_cov: Extract the covariance matrix based on the information of mf=34. It will go in a separate branch.

_get_edistr_cov: Extract the covariance matrix based on the information of mf=35. It will go in a separate branch.

get_cov: Get it in a dictionary {mf: CategoryCov}. If only one mf exists, it automatically returns the CategoryCov object. To test it, tests identical to _get_xs_cov were used. Once the branch https://github.com/AitorBengoechea/sandy/tree/errorr_get_cov is in sandy, I will proceed to test nubar and cov with processing.

[AitorBengoechea]

As the functions _get_lpc_cov and _get_edistr_cov are created and the branch https://github.com/AitorBengoechea/sandy/tree/errorr_get_cov accepted, more tests will be carried out for different situations.

[AitorBengoechea]

Priorities for new sampling:

1. https://github.com/AitorBengoechea/sandy/tree/errorr_get_cov

2. https://github.com/AitorBengoechea/sandy/tree/endf_get_cov

3. I will upload the script where I have stored the new sampling.py.

4. For the perturbation in the sampling, the need to accept the https://github.com/AitorBengoechea/sandy/tree/fy branch

[AitorBengoechea]

With the new pull request the order of priorities is as follows:
Priorities for new sampling:

1. https://github.com/AitorBengoechea/sandy/tree/errorr_get_cov

2. https://github.com/AitorBengoechea/sandy/tree/get_lpc_cov

3. https://github.com/AitorBengoechea/sandy/tree/get_edistr_cov

4. https://github.com/AitorBengoechea/sandy/tree/endf_get_cov

5. I will upload the script where I have stored the new sampling.py.This script will create the samples without disturbing them.

6. For the perturbation in the sampling, the need to accept the https://github.com/AitorBengoechea/sandy/tree/fy branch

7. Review of the custom_perturbation of the other classes

8. Perturb the samples and the perturbed samples obtention