Allows automatic increasing of Nu given negative elements in f0.

Modifies MultiBolt to automatically increase Nu given negative elements detected in f0, which may indicate negative transport coefficients will be output, mostly seen at high E/N. This option can be turned on and off using similar methods to the energy remapper (--USE_NU_REMAP or p.USE_NU_REMAP = true/false). Additionally, the max Nu value the loop will test before giving up (Nu_max) and the increase in Nu per iteration (Nu_iter) can be changed on the input as well, as optimal settings can vary.