Castep io write velocities into .cell file

for #945

I have implemented the code for writing and struggled to write a correct unittest for it, since there is no castep installed in the testing suite (locally yes) but this does not require more than recognition of some keywords. How should I write the test? Perhaps if I have created a central fixture that acts as a dummy keyword set, copied from the test/calculator/castep/ directory's one test file that could be used, but how here since Castep() calculators are created only under the hood somewhere.

Moreover, mypy fails on the code as well, though I see that in general on the castep IO module and not on my exact new code, though in a function that I have touched. Is it only checking parts that we have touched in a given branch?