WIP: Add Powheg, 21.6 branch (2019.03.13.)

This is meant as a demonstration for the generators group on how to use the Powheg tgz archive file(s) put on EOS. At least how I imagine that they should be used...

So let me just ping everybody who's on the e-mail list on which it's discussed since a while. @ewelina, @dhayden, @wlampl, @disipio, @fsiegert, @chays, @amoroso, @strichte, @obreshko.

The idea with this External/Powheg package is the same as with External/dcap. It just downloads a binary tarball archive that it then puts into the release. I chose to put things under <release dir>/POWHEG-BOX. I know that the tarball itself uses this naming, but because of how ExternalProject_Add works, one needs to be explicit about it in the CMake code.

Then I just updated the Generators/PowhegControl package to set the $POWHEGPATH environment variable to point at this path inside the release.

Compiling these two packages together using Projects/WorkDir should put you in an environment in which these test binaries can actually be tested. So please go ahead and experiment with this setup a bit. And eventually open your own merge request along these lines.