Add helper functions for reals in typeconverter
Add helper functions for reals in typeconverter

Added:
    - postive_real
    - nonnegative_real

These functions check for numbers that lie within their respective sets.

joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue