move Parametrization to a separate header

enclose the enum within namespace+struct; provide methods to validate
and to convert to string.
move Systematic enum to separate file
drop unnecessary namespace specifiers
major refactoring

Initialize specifying the names of the signal regions.  This allows to
get rid of the enum `Region`, which caused so much headeache. Now the
rates are simply accessed with an index region.  If the user sees that
`getIndexRegion` is the bottleneck, then he/she should cache it.
This solves #2
Merge pull request #9 from gerbaudo/refactor

Refactor