negative signal values unaccounted for

[drbergman]

Negative signal values have unexpected effects on rules depending on the hill power:

• even ==> sign is effectively ignored ==> treat extremely low values (<0) as their positive opposites
• odd ==> introduces negatives into the multivariate hill function inputs, possibly causing negative outputs (with unintended consequences) or even a divide-by-zero error
• non-integer ==> produce nan results

Note: Non-integer hill powers are allowed by PhysiCell (though not Studio) as the parser is std::atof and not std::atoi

For the most part, this is ok since most signals are non-negative. However, intracellular concentrations can go negative and often do. We could either address this when getting the intracellular signal (max that return with 0) or set a check at rule evaluation, incurring extra cost across all rules/signals.