First step in refactoring of arcslideda.py.

Isolated extension by identity into the extendbyid module. Note the
getDAStructure function in arcslideda is kept as a redundancy check.
There are currently two limitations to extension by identity: generators
in the local bimodule must have unique idempotents, and there can be at
most one single point in each local PMC.
Second step in refactoring arcslideda.py.

Added function (in dastructure.py) to convert any DA structure with
delta and deltaPrefix to a simple DA structure (infinite number of
terms in the action is not allowed).

Removed some unnecessary functions. Removed testDeltaAndPrefix because
there is now no redundancy (simple DA structure obtained directly from
delta and deltaPrefix).