Add IntercalEdge table

IntercalEdge is essentially a directed graph containing zeropoint
offsets between connected fields of a single bandpass.

Add edge_exists staticmethod to IntercalEdge table
Add intercal prepare_network
Compute field-to-field zeropoint differences
Solve network of field-to-field zeropoint corrections
Set/unset function for intercal ZP reference metadata
Alembic: add intercal_edge table