Added fmt as external dependency and printing example

[lobis]

Added fmt (https://github.com/fmtlib/fmt) as dependency to help with string formatting.

I also moved -lstdc++fs outside of that block, I think it was a mistake (it should always be linked).

I added some simple class / methods in the fmt namespace and placed in TRestTools.h/cpp to help with printing.

Methods such as fmt::ToEnergyString take a number in the default REST energy scale and return the appropiate string, for example fmt::ToEnergyString(10000) returns "10.00 MeV". Other methods such as fmt::ToLengthString have been added and its easy to implement more.

I updated the print for TRestEvent (and TRestGeant4Event/Track in another PR) to showcase what can be done with this library.

For instance one can define a custom formatter for any class, such as TVector3 to simplify printing:

template <>
struct fmt::formatter<TVector3> : formatter<string> {
    auto format(TVector3 c, format_context& ctx) {
        string s = fmt::format("({:0.3f}, {:0.3f}, {:0.3f})", c.X(), c.Y(), c.Z());
        return formatter<string>::format(s, ctx);
    }
};

TVector3 myVector = {1,2,3};
fmt::print("{}", myVector); // will print "(1.000, 2.000, 3.000)"

Its very easy to add colors to the terminal which helps when reading the output:

[nkx111]

Is this dependence necessary?

We already have ToString() method for TVector3, through TRestReflector, and colored output methods in TRestStringOutput. We can also modify this class for energy output methods.

If fmt library is really better, I suggest to modify TRestStringOutput and try to adapt it. For example, we can make RESTMetadata<<"xxx"<<RESTEndl; calling print(fg(color::green) | emphasis::bold, "xxx"); internally.

[jgalan]

I think it is good that we keep a unified way to output things on screen. I.e. using REST_xyz macros, I think that is the point from @nkx111 in the last comment.