Solves problem when course components have non-numeric values for grade-weight

Discovered in assisting an edx/solutions customer. Some xblocks publish weight as a non-numeric value, so if we are to use it to weight the grade for the problem, it needs to be cast as float in order to calculate the weight correctly.

Certainly seems like something that could be addressed by the xblock developer, but if we want to encourage more xblock development, it is probably good to be more resilient.