Fix return value
Add `TransformationMatrix` class

Add `TransformationMatrix` class and a function to compute the
transformation between any two frames - `get_transformation_matrix()`.
Add unittest module for `get_transformation_matrix()`.
Add `pysymoro/tests/transformationmatrix.py` file.
Update `pysymoro/tests/run_all_tests.py` to discover all the unittest
modules in the folder and run the tests.