Non-MPI build broken

Describe the bug
Non-MPI build is broken and not tested in CI.

To Reproduce
Steps to reproduce the behavior:

1. Configure with -DENABLE_MPI=OFF
2. Build geosx
3. See errors

/home/klevtsov/work/GEOSX-develop/src/coreComponents/common/MpiWrapper.hpp:826:38: error: unused parameter 'buf' [-Werror,-Wunused-parameter]
int MpiWrapper::recv( array1d< T > & buf,
                                     ^
/home/klevtsov/work/GEOSX-develop/src/coreComponents/common/MpiWrapper.hpp:828:27: error: unused parameter 'tag' [-Werror,-Wunused-parameter]
                      int tag,
                          ^
/home/klevtsov/work/GEOSX-develop/src/coreComponents/common/MpiWrapper.hpp:855:55: error: unused parameter 'buf' [-Werror,-Wunused-parameter]
int MpiWrapper::iSend( arrayView1d< T const > const & buf,
                                                      ^
/home/klevtsov/work/GEOSX-develop/src/coreComponents/common/MpiWrapper.hpp:857:28: error: unused parameter 'tag' [-Werror,-Wunused-parameter]
                       int tag,
                           ^
/home/klevtsov/work/GEOSX-develop/src/coreComponents/common/MpiWrapper.hpp:906:34: error: unused parameter 'value' [-Werror,-Wunused-parameter]
U MpiWrapper::prefixSum( T const value, MPI_Comm comm )
                                 ^
/home/klevtsov/work/GEOSX-develop/src/coreComponents/common/MpiWrapper.hpp:906:50: error: unused parameter 'comm' [-Werror,-Wunused-parameter]
U MpiWrapper::prefixSum( T const value, MPI_Comm comm )
                                                 ^
/home/klevtsov/work/GEOSX-develop/src/coreComponents/common/MpiWrapper.hpp:925:82: error: unused parameter 'comm' [-Werror,-Wunused-parameter]
T MpiWrapper::reduce( T const & value, Reduction const MPI_PARAM( op ), MPI_Comm comm )

... etc.

Expected behavior

• Non-MPI build should work
• Non-MPI build should be tested in at least one CI configuration

Alternatively
ENABLE_MPI option should be deprecated and removed.