Resolve "Add function to insert assets, to group assert insertion"

Two functions are added to exposure-lib: the insert_assets function in the AbstractExposure database, that handles the standard way of inserting assets (iterating over each asset and adding each with an insert statement). In the PostGISExposure database, the insert_assets function is overwritten to use the mogrify function provided by psycopg2, that inserts all assets at once.

Closes #66 (closed)

\approve @ds @tara @shinde