output prefix METplus configuration

[georgemccabe]

I added <APP_NAME>_OUTPUT_PREFIX (i.e. GRID_STAT_OUTPUT_PREFIX) in the METplus config for all MET tools that have output_prefix in their MET config files. This was done so users can configure their output prefix from the METplus config file instead of having to go into the MET config file and modify the variable.

Previously a few wrappers (GridStat, MODE, and MTD) set environment variables that were referenced in the MET config file output_prefix, like FCST_VAR, OBS_VAR, and LEVEL (which was always forecast level but unclear that it was). In these wrappers, the following METplus config variables are set for each iteration of the fcst/obs name/level loop:
CURRENT_FCST_NAME
CURRENT_FCST_LEVEL
CURRENT_OBS_NAME
CURRENT_OBS_LEVEL

These can be referenced in the [GRID_STAT/MODE/MTD]OUTPUT_PREFIX, i.e.
MODE_OUTPUT_PREFIX = {MODEL}{CURRENT_FCST_NAME}vs{OBTYPE}{CURRENT_OBS_NAME}{CURRENT_OBS_LEVEL}