Add a logging system that records and prints warnings

[cschwan]

Related Issues:

• https://github.com/NNPDF/pineappl/issues/253

[alecandido]

Not exactly logging, but you could consider tracing (tracing is a bit more than logging, but it might be useful in PineAPPL, e.g. to debug and profile new interfaces)

[cschwan]

In the meantime I think warnings aren't a good idea anymore. An operation should either succeed or fail.

[alecandido]

I arrived more or less to the same conclusion: warnings are mostly cluttering the output and ignored. Instead, if you want to pass some further information down to your user, you should add that to the function output, not printing it on stdout.