tests/db/model_hzrd_parsedsource_test:

Removed; these test are related to some code which is about to be removed.
db:

Removed `polygon` column (representing the rupture enclosing polygon) from
`parsed_source` table.
input/source:

SourceDBWriter no longer computes the rupture enclosing polygon of each
source. This has never been used and we do not inticipate that it will
be needed.
tests/input/source_test:

Removed part of a test related to the storage of the rupture enclosing polygon
for a `ParsedSource`. This functionality wasn't used and has been removed.
db/models:

Added `characteristic` to `source_type` choices of ParsedSource.
db/schema/openquake.sql:

Added `characteristic` to allowed values of `parsed_source.source_type`.
input/source:

Added `characteristic` to `_source_type`.
tests/data/mixed_source_model.xml:

Added characteristic source examples (3) to test file.
input/source:

Added util function to convert characteristic source from nrml to hazlib.
tests/input/source_test:

Sketched 3 test cases for characteristic fault source conversion.

The `simple` variation already passes.
tests/input/source_test:

Finished complex characteristic fault conversion case.
tests/input/source_test:

Finished multi surface characteristic fault source conversion test case.