Make awg prefer TARGET environment variable and clean up behavior WRT param paths and model rates.

Modify the behavior of agtpman with regard to target directories and model rates as follows:

awgtpman now prefers to use the TARGET environment variable for the target directory.
If not present, /opt/rtcds/<site>/<ifo> will be used.
If neither TARGET nor SITE and IFO are set, awgtpman will refuse to load
and print a helpful error.

awgtpman will look in $TARGET/target/gds/param for the parameter files
to get the model rate.
awgtpman will now fail with a helpful error when the model rate cannot be found, unless '-r' option is set.

If the '-r' option is set, then awgtpman will use exclusively the command line arguments to
determine model rate and won't check the param files.
awgtpman will ignore command line arguments for setting the model rate unless '-r' is given.