Python 2.7 compatibility and testing

[mrosemeier]

Since some users still rely on Python 2.7 written code it would make sense to provide also 2.7 compatibility for pycalculix, especially when many different tools are coupled into optimization frameworks such as OpenMDAO (<1) it is required to bring all codes down to a common denominator.

I don't think this would be too much effort. However, I would also suggest to add some unit testing.

[spacether]

I don't plan on porting the project to python 2.7