fix prod calculation for random output amounts with catalyst specified

[kalenedrael]

Productivity only applies to amounts above the catalyst amount. When the productivity bar fills, a set of outputs (random or not) is generated and the catalyst amount is subtracted from each output, with a minimum of 0 (no negative outputs). This fixes the calculation of the "proddable amount" - the effective amount of output that prod can apply to.

This behavior was tested using this mod. Compare with expected and actual results in this spreadsheet.

Fixes issue #88.