(PyKDL) expose ChainExternalWrenchEstimator

[NRRHHH]

Also add .gitignore.

[MatthijsBurgh]

Please also copy the relevant tests from cpp to python

[MatthijsBurgh]

Please create a separate commit for the .gitignore file

[PeterBowman]

Please create a separate commit for the .gitignore file

Done.

Please also copy the relevant tests from cpp to python

This might take some time since tests for ChainExternalWrenchEstimator are coupled with other tests which are also missing.

[MatthijsBurgh]

@PeterBowman Are you planning to add the tests? I don't want to add features without adding tests.

[PeterBowman]

@MatthijsBurgh sure, I was a bit busy, next week I'll get back at this. As I noted before, this class is coupled with several other solvers in solvertest.cpp, which are currently not covered in kinfamtest.py. In order to translate ChainExternalWrenchEstimator's C++ test cases to Python, it seems to be necessary to add tests for those other classes, too. Is this correct?

[MatthijsBurgh]

When a test case is repeated for multiple solvers, it would be preferable to copy them all to python.