Implement pytests

Implement pytest unit tests for the functions in multiprecision.cc

Some tests currently fail. They are marked with the pytests.mark.xfail function decorator. At some point (in this MR or later) the failures will have to be adressed, either by removing/modifying the tests or by changing the behaviour of the tested functions. Remove the decorator to see error message when running tests.

Currently there are 202 tests running. This is probably more than strictly needed, mostly because parametrized tests are run on many parameters, but they run quickly anyway, so it is maybe not a big problem now. If needed some tests or test parameters can be cut.