make functions for compute time, output size more general

[danielskatz]

Rather than using predefined formula in the application description, could we use a python line of code that skeleton evaluates?

currently, compute time (T) can be a polynomial function of input file size (I): T = a * I ^ b
and output file size (O) can be a polynomial function of input file size (I) or compute time (T): O = a * I ^ b or O = a * T ^ b

(a suggestion from @mjwilde, so to be assigned to him :) )