Changes v2.0.0 to v2.1.0

This is primarily a bugfix and cleanup release. The build system got a nice cleanup and the API didn't change much, if at all. An incomplete set of changes:

• Internally used casadi.py was renamed casadi_loader.py. This should not affect any end-user python code, but if you build from source uncleanly or install to a location that you specified with earlier versions, there may be a lingering casadi.py or casadi.pyc that spoils the installation. Remove it.

• add the ability to use system packages OR packages in external_packages (#1108)

• lots of sundials cleanup (#1198, #1231, #1235, #1236)

The full list of changes is here