Improve tmpdir usage

[gdevenyi]

1. add the synthstrip_N3 and input filename to tmpdir naming
2. Add the function name to the mktemp functions
3. Re-define TMPDIR in the script so that the temporary directories from functions are nested in the top-level tmpdir

[CMonnin]

Resolved by #7 and subsequent tmpdir work. All three items are implemented in synthstrip_N3.sh:

1. Script + input filename in the top-level tmpdir name (line 462: mktemp -d --tmpdir "tmp.${__base}_${input_base}_XXXXXX")
2. Function name in per-function mktemps (lines 621, 642: mktemp -d --tmpdir "${FUNCNAME[0]}_...")
3. TMPDIR redefined so function temp dirs nest under the top-level tmpdir (line 463: export TMPDIR="${tmpdir}")