set MPAS_DOUBLE_PRECISION to OFF

[guoqing-noaa]

currently, MPAS_DOUBLE_PRECISION is set to ON

if(MPAS_DYCORE)
  set(MPAS_DOUBLE_PRECISION "ON" CACHE STRING "MPAS-Model: Use double precision 64-bit Floating point.")
   ....
endif()

This will increase file size. compared to single_precision.
Per communications with NCAR folks, single_precision is good enough.
It's time to consider a transition to single-precision