Remove metapop

[molpopgen]

Address #109 by complete removal of metapopulation code "like a bandaid".

The distinction between a "population" and a "metapopulation" is merely one of book-keeping, and should not require a separate pop type + supporting code.

• [x] update pop tag names to reflect locus, not deme
• [x] update pop type names to reflect locus, not deme
• [x] rename pop type files
• [x] rename unit test files and types
• [x] Update all doxygen strings
• [x] Update any relevant markdown documenation