Update nrtest comparison tolerance parameterization

[michaeltryby]

Comparison tolerances are an important, but often overlooked, aspect of numerical regression testing. There is a need to associate comparison tolerance parameters with specific comparison routines and tests to increase flexibility when performing comparisons.

The python package nrtest currently only allows one type of parameterization and it can only be modified globally. Therefore, specific changes to nrtest are needed to:

1. Allow different comparison tolerance parameters for different comparison routines
2. Associate comparison tolerance parameter values with specific tests
3. Maintain the ability to modify comparison tolerance parameters globally

These modifications would enhance the value of nrtest for the OWA/EPANET and SWMM projects. See nrtest Issue #15