add support for building/installing NAMD

This is a cleanup/extended version of PR #354 by @georgets .

Besides style cleanup, it also:

 * automates/abstracts the configuration options (e.g. Charm version, compiler part of target architecture, FFTW/CUDA config flags, ...)
 * runs a test case during the build
 * includes a reasonable sanity check

This results is cleaner easyconfig files (cfr. https://github.com/hpcugent/easybuild-easyconfigs/pull/835).