State memory handling

[mattldawson]

After moving temporary solver variables to the State class, we should consider options for optionally allocating/deallocating these temporary variables.

This could include:

• the ability to create "minimal" instances of the Matrix classes (without index arrays for the sparse matrices and without host memory for CUDA matrices, e.g.)
• maybe a function on the State class to allocate/deallocate all temporary variables to reduce the memory footprint between calls to Solve.

[sjsprecious]

According to the discussion with @mattldawson , one way to do this is to create another simple constructor for the CudaDenseMatrix and VectorMatrix, which only allocates the memory space of a given dimension matrix without allocating additional index arrays. The constructor may look like MatrixType(Bool minimal_instance, int x_dim, int y_dim)?

[sjsprecious]

Also we can implement a separate CleanUp function in the State class to manually deallocate the temporary variables if necessary.