Use explicit host-device syncing

[pelesh]

Instead of having matrix getters (e.g. Csr::getValues()) sync host-device data under the hood, perhaps we should require explicit sync calls. So instead of

  // allocate and initialize matrix A on host
  double* a = A.getValues(memory::DEVICE); // getValues copies values from host to device

one would do

  // allocate and initialize matrix A on host
  A.syncData(memory::DEVICE); // explicitly copy matrix data  from host to device
  double* a = A.getData(memory::DEVICE);

Here are a few arguments why this might be a good idea:

• The code is more readable, the intent to perform operation in specific memory space is made explicit.
• Current implementation silently ignores syncing if the source memory space is not up-to-date, leaving user under impression that the syncing was successful.
• Re::Solve's solvers are really not using implicit syncing. It is only tests and examples that used this features.
• With getters not modifying matrix class by syncing host and device memory, we can implement const getters with identical behavior. That could help address Consider using const in ReSolve code more aggressively #89 more effectively.