Symbolic Pofk implementation as boltzmann solver

[santiagocasas]

Closes #67

Changes proposed in this pull request:

• symbolic class as a solver in cosmology and config.
• new yaml files for symbolic
• removing hard-coded neutrino parametrizations
• improving feedback prints
• complete docstring documentation of cosmology class
• minor fixes in related classes

Before submitting

• [ x] I've read and followed all steps in the Making a pull request
  section of the CONTRIBUTING docs.
• [ x] I've updated or added any relevant docstrings following the syntax described in the
  Writing docstrings section of the CONTRIBUTING docs.
• [ x] If this PR fixes a bug, I've added a test that will fail without my fix.
• [x ] If this PR adds a new feature, I've added tests that sufficiently cover my new functionality.

[Sefa76]

Very nice work thank you! Just small optimizations and a fail save

[santiagocasas]

Thanks for the review ! I have committed changes so that this can be merged!

[Sefa76]

Approved. Thank you!