Complete implementation of T Model functions in demography module and test better.

[davidorme]

Is your feature request related to a problem? Please describe.

The most recent code (PR #282) include only some of the T Model functions and the testing is against a small number of hand calculated values.

Describe the solution you'd like

Complete the transfer of T Model functions from within the to be deprecated pyrealm.tmodel module and then build a deeper regression test suite against the R T model implementation.