Add support for meshes in device memory in MFEMSidreDataCollection

[aaroncblack]

The Marbl code has test problems where the MFEM mesh data resides in device memory and has calls to the MFEMSidreDataCollection SetMesh() function.

The data collection source currently expects the MFEM mesh grid functions to reside on the CPU, as it is using the grid functions 'GetData()' to get the pointers to the data. These will return invalid host pointers if the grid function data is on the GPU.

I believe the source can be updated to support the MFEM mesh GF data in either cpu or gpu memory by replacing GetData() with HostRead() or HostReadWrite() calls to migrate the data if needed.