Multiprocessing, averaging

do period search with multiprocessing, replace weighted variance calculation for loops with numpy.average, value exceptions for numc.
I prefer spaces around the operators for statements like this. This may not be your personal style but for patches like this it's nice to try to adopt the style of the preexisting code for consistency. Note also the spaces before the parentheses in function calls ... which is a style that very few people use, I'll admit.