Eliminate redundant function initialization

Currently in Unicorn unstable we must manually construct the function spaces that the coefficients will reside in. We must then manually keep track of which coefficient resides in which function space. This raises the possibility of specifying one function space in function initialization, while a different function space is specified in the form file. I suggest we elinminate this redundancy and possible cause of bugs by initializing the coefficients in the FormFactory, where the corresponding function spaces are known for each coefficient (the form knows this via eg. BilinearForm::CoefficientSpace_foo).