cuda linear algebra support

[martinjrobins]

TODO:

• add a Context struct to all vectors/matrices/operators that can carry the details of the execution/allocation context ( e.g. the device and stream for cuda stuff, number of threads for cpu stuff)
• add a set of vector structs that use cuda to allocate vectors on the gpu and implement the standard Vector traits (use cuBLAS)
• add a similar set of cuda matrix structs for dense matrices (use cuBLAS)
• add a cuda sparse matric struct (use cuda vectors and implement a CSC matrix)
• add a dense cuda linear solver (use cuSOLVER)
• add a sparse cuda linear solver (use cuSOLVER)

[martinjrobins]

added dense solver in #147