doctests updated for new 9-digit Quantity().__repr__()

added dependency on Serialize to requirements_docs.txt

in systems.rst: three errors remain, one may be a regression,
the others I'm not qualified to comment on

in nonmult.rst: some issues around floating point estimation are made
more robust by using print()

seems like Quantity().__repr__() changed since these
tests were written, and could benefit from some floating-point
truncation

in pitheorem.rst the only error was dictionary ordering, which
is now deterministic in python3.7, but may still cause
issues down the line

no doctests in getting.rst, marked all as code blocks

The nullspace of the dimensionality matrix is now obtained without NumPy assistance.
This eliminates NumPy as a requirement to use the Pi Theorem. Moreover, the implemented
gaussian elimination uses fractions (not floats) by default which is more proper for
the type of matrices usually found in this application.