revisiting the handling of variations

Part of the MR has been extracted from !127 and the comments related to the revised handling of variations.

Ths main change is the introduction of Boost.Spirit to parse expressions such as Unc[1-10]{Up,Down}. This supersedes the enum VarType (now removed). The doxygen of MetaInfo::AddVars provides examples.

Also includes all developments of plugins that are still missing in dev branch.