subset_data: "For running this case" message needs better paths

After running `subset_data`, the user gets a message like this:
```
For running this regional case with the created user_mods :
./create_newcase --case case --res CLM_USRDAT --compset I2000Clm60BgcCrop --run-unsupported --user-mods-dir subset_data_regional.crop/user_mods
```

There are two issues here:
- It always says `./create_newcase`, regardless of where the user is in the directory structure. This should be changed so that it gives the correct path from wherever the user is.
- The path to the usermods directory should be absolute, just to be safe.