Added an evaluate_expr function, that will calculate the value of 'A+B' for example, given the current values of A and B in the network. Cleans up a lot of the evals that were happening before.
Added an evaluate_expr function, that will calculate the value of 'A+B' for example, given the current values of A and B in the network. Cleans up a lot of the evals that were happening before.

GutenkunstLab/SloppyCell,GutenkunstLab/SloppyCell