Feature/convtest
This implements the convergence test interface. A ctest checking the interface is added to our test suite (it also demonstrates how it's supposed to work). As well as a python test for the convergence test info extractor module. The ini syntax mentioned in issue #30 (closed) is not fully implemented yet, now it reads the following keys:
__output_extension = output # this is the default if not specified
[__CONVERGENCE_TEST]
ExpectedRate = 1
AbsoluteDiff = 0.1
TestKey = Grid.Level