migrate to 'fat' easyconfigs

[boegel]

We should migrate this repository to host 'fat' easyconfigs, which specify build parameters for different versions/toolchains for one particular software package, maybe in YAML syntax (see also easybuilders/easybuild-framework#1324).

The EasyBuild framework can be enhanced such that running eb WRF-3.6.1-intel-2015a-dmpar.eb makes EasyBuild go look for WRF.eb first to see if the specified build parameters can be fulfilled using that 'fat' easyconfig file.

[boegel]

I hope @Caylo can spend some time on implementing the support required to make this happen...