Fix usage of Semigroup/Monoid instance of Header

[ocramz]

Frames are not supposed to be constructed directly by the user (they should only be 'encode'd from data).
Currently we say 'hdr = mempty' in a few places for convenience (grep for all FIXMEs) but these should become a function of either inputs

For example, the header of a join of two Frames is the outer product of headers (?) -- look up schema computation in relational algebra/SQL

Blocks #11