algebras checked in the front-end

As of revision 769, MxAlgebra are checked for correct dimensions before the optimizer is called.

> A <- mxMatrix("Full", nrow = 1, ncol = 1, name = 'A')
> B <- mxMatrix("Full", nrow = 1, ncol = 2, name = 'B')
> C <- mxAlgebra(A + B, name = 'C')
> model <- mxModel('model', A, B, C)
> mxRun(model)
Running model 
Error: The algebra 'model.C' in model 'model' generated the error message:
non-conformable arrays