Update ConjGrad StdMat operator implementation for MPI

Field.hpp

MemoryRegionCPU.hpp

MemoryRegionCUDA.hpp

Operators/ConjGrad/ConjGradStdMat.hpp

Operators/FwdTrans/FwdTransStdMat.hpp

Operators/HelmSolve/HelmSolveStdMat.hpp

Operators/Matrix/MatrixCUDA.hpp

Operators/Matrix/MatrixStdMat.hpp

Operators/NullPrecon/NullPreconCUDA.hpp

Operators/Operator.hpp

Operators/OperatorConjGrad.hpp

tests/CMakeLists.txt

tests/init_diagpreconfields.hpp

tests/init_fields.hpp

tests/init_fwdtransfields.hpp

tests/init_helmsolvefields.hpp

tests/init_nullpreconfields.hpp

tests/test_nullprecon.cpp