Gpu support

[dev10110]

Allows constructing a STGPKFProblem where all the large matrices live on an Nvidia GPU.

mostly a pass through of CuArray instead of Array , but because of fairly extensive use of Kronecker products in specific matrices, it requires some special implementations of Kronecker products to work on the GPU.

The CUDA requirement is added in as a weak dependency (i.e., as a package extension) so it should only be loaded if the user has loaded the CUDA package.

The main entry point is CudaSTGPKFProblem instead of STGPKFProblem - the rest of it should work as before.

There is a breaking change on the type heirarchy, where before STGPKFProblem{P, F, ...} has been replaced with STGPKFProblem{F, P} where F refers to the float-type, and P refers to the state-type of the problem domain.