Increased upfront validation

I've written a Validator class that I will add to the next pull request. At the moment, it is unimplemented. This class can do simple upfront checks such as ensuring source and target are of the same shape type, ensuring the layerfile input isn't broken, as well as anything else really you might want to check.

I initially was going to implement it into the python toolbox parameters, but the validation process takes too long to complete while still providing a satisfying user experience, so for the moment, it has been scrapped. I'll look to add them into the runtime scripts instead in the near future.