Enable use of alternative yaml implementations

[tclune]

Currently pFlogger depends explicitly on yaFyaml. This is nice in terms of reducing dependencies on packages outside of GFE, but yaFyaml remains fragile due to various Fortran compiler issues.

Many of my projects already use ESMF's HConfig layer to use yaml, and it would be nice if there was a way to have pFlogger use that package either as a compile-time option, or even better as some sort of dependency injection.

Currently LoggerManager has load_file() which uses yaFyaml internally. But an alternate implementation of load_file() should be possible presuming that all the necessary interfaces are public. I think this is the case.

Needs a bit of thought.

[tclune]

I think that if we create a factory method elements = make_ConfigElements() in Config.F90 then LoggerManager could use that factory method and not explicitly depend on yafyaml. The constructor for LoggerManager could then have an optional argument that is a factory method to use.