Organize ERF source code into a library

[rafmudaf]

Reorganize the CMake project and Source directory so that ERF is compiled into library and this is linked to any executables

[rafmudaf]

The work I did to reorganize the ERF source code into a library is in this branch: https://github.com/rafmudaf/erf/tree/add_testing. See the last few commits to understand the process and progress.
cc @rfhaque

[dwillcox]

A general setup that can run most problems would make the initialization more elegant.

Could benefit from the code parser Weiqun wrote for WarpX: https://github.com/WeiqunZhang/WarpXParser

[asalmgren]

@dwillcox -- My understanding is that this issue is completely independent (at this point) of using the parser -- this doesn't change anything about the problem setups...

[asalmgren]

This was completed by Jean Sexton