Fixes #748

Description

The unit tests and examples were failing when run against the latest MODFLOW6 nightly. This is caused by a change on the MODFLOW6 side were any gwf-gwt exchanges has to have an actual existing file (even though it is emty).
The precious behaviour was that just mentioning the gwf-gwt exchanges in the nam file was enough.

This commit adds the gwfgwt package. It uses the same logic as the gwfgwf exchanges in terms of how it is written to disk.
Any common methods between the different type of exchanges has been moved to the newly created ExchangeBase class

Note
I didn't add any unit tests for the simulation class. The reason for this is twofold:

• I moved code around, no major functional changes
• We have to refactor they way the gwf-gwt exchanges are being created the moment we start working on the multimodel support for gwt, making any unit tests I add obsolete

Checklist

• Links to correct issue
• Update changelog, if changes affect users
• PR title starts with Issue #nr, e.g. Issue #737
• Unit tests were added
• If feature added: Added/extended example