PolyChord + Proposal

[JesusTorrado]

TO-DO list (installation left for the end of the PR, to have a working prototype asap):

• J: Incorporate means to covmat database, and create way to retrieve them.
• J: Make Polychord inherit from CovmatSampler, and add necessary modifications.
• A: Merge modifications into new code.
• A: Create a test.
• J: Have install and is_installed take full input info, to check for new dependency.
• J: Have bib take full input info, to check for new citation info. [Done in bibtex PR -- need yet to apply to this case]
• A: Make it use a subset of parameters (generate a dummy covmat

@appetrosyan Feel free to add more detail to the to-do list.