Use nnpdf single-truth theory

[scarlehoff]

Changes to use NNPDF/nnpdf#2028 with pineko.

At the moment it doesn't do anything clever, it just loads the yaml from there. I've tested the eko and fktable generation.
(the tests and so won't pass until the PR is merged in nnpdf)

[scarlehoff]

I've made a change (wrt #146). Now if the template and the theory have a different mu0 this raises an Exception instead of overwriting the value silently.
I can revert the change back of course, but I think this is something important enough to be fail as early as possible (and the solution is "simply" to change the template or theory if one really wants to play god).

Note, the benchmarks have changed because I'm putting theory > template but maybe we want the template to take over?

[scarlehoff]

One problem here is the nfonnl part.
However, I believe that, since now one can run nfonll by providing only the main theory id, the only logical thing is that the theory cards can also be autogenerated in the go (and so tcards should not be necessary).

This helps reducing the extra amount of theory cards and having to tell NNPDF about things like PTOEKO or FONLLParts.

[scarlehoff]

With the last commit from here NNPDF/nnpdf#2067
this works with pineko to create theories without having to provide a theory or yamldb folder.

[giacomomagni]

I left two two useless comments.

[felixhekhorn]

I dropped most of the yamldb stuff, because it is not used here and nowhere else