Version 2.6.1

Major changes:

• Introduce the Client Simulator (see the README). This is a framework for integration tests. It has its own CMake project - if you want to install it, run the usual cmake/make invocations in tools/clientSimulator.
• Wrote a functional-tests plan and started to implement some of them. This new test suite uses the (native) client interface and will, at some point, replace the previous test suite which was using the remote interface.

Less important changes:

• Fixed an infinite loop during plugin loading. It happened when a plugin was not correctly presenting its entry point.
• Criteria: check for duplicate criterion literal values (only duplicate numerical values were checked so far); return an error to the client.
• Correctly forward libxml2 error messages to the client instead of dropping them.

Misc:

• Doxygen fixes
• Some dead code removal
• CI: Coverage fixes