update xgrid unit test to write out fields after remapping

[uturuncoglu]

This PR extends ESMF_XGridUTest to write out the fields after interpolation.

• The test can be run as following after building ESMF (you might not be run build again),

cd src/Infrastructure/XGrid/tests
make build_unit_tests
make run_unit_tests
make run_unit_tests_uni

Note: you could use following command to create interactive session on Cheyenne,

qinteractive -l walltime=21600 -l select=1:ncpus=18:mpiprocs=18

• Fields are named as dstField1.nc and xField1.nc for 1 PET and dstField2.nc and xField2.nc for 2 PET cases.
• Then, files can be compared with cprnc as following,

cprnc -m xField1.nc xField2.nc
cprnc -m dstField1.nc dstField2.nc

Note: cprnc tool can be found in /glade/p/cesmdata/cseg/tools/cime/tools/cprnc/cprnc