Rename
Add `plot_grd_file.rb'
Enhance readability
Add `--cpt' option
Improve `usage_and_exit' so that usage can be put into stderr when exit_status is not zero
Fix help messages
Change output from eps to sh

Usage:
  Before: plot_grd_file.rb -f GRD_FILE > tmp.eps
  After: plot_grd_file.rb -f GRD_FILE | sh > tmp.eps
Delete an useless line
Improve reusability of output shell script