Start to modularize plotting scripts in reg_tests/

Added reg_tests/common/plot_settings.ncl which contains NCL routines to set
common plotting resources (gsnDraw and gsnFrame both False, set min and max
values for both axes, and set the function code to ":"), then updated all the
plot_diff_coeffs.ncl routines to make use of this new module.

Also removed the leading space from attributes that are written to netCDF files
(noticeable in the global "column_lat" and "column_lon" attributes of the
single column tidal output).