Adding MPFR library at the framework level

[jgalan]

This PR simply brings the MPFR library to the framework level and not only for axionlib.

MPFR library allows calculations with any required number precision.

I will soon use it with a new class TRestComplex to do calculations with exact precision, but for the moment this change is required by the recent rest-for-physics/axionlib#12 PR.

[jgalan]

Plz @lobis could you check it is the appropriate way to introduce this inside CMakeLists?

[lobis]

Plz @lobis could you check it is the appropriate way to introduce this inside CMakeLists?

I made some modifications.

Instead of throwing an error when not found I set it to warning, I don't think we want to give an error when this library is not found right?

The previous implementation relied on the MPFR_PATH variable to exist, otherwise the call to link_directories would not work even if the library was in your system. Now it should work in more cases but I have not tested it.

In my machine using the pipeline docker image I don't need to specify this MPFR_PATH variable for it to work.

Maybe the link_directories step can be skipped, not sure.

[jgalan]

Great addons @lobis, lets then approve and merge the changes?

[nkx111]

We should add some flags if this library is not linked, then one can type rest-config --flags to see if he can compile a MPFR-dependent REST library(like axionlib) based on this installation of REST