Implement environment variable REFL1D_JOB_SUB_DIR.
This fixes https://code.ornl.gov/sns-hfir-scse/reflectometry/reflectometry/-/issues/284.
The work is to use REFL1D_JOB_SUB_DIR
to replace hard coded reflectivity_fit
in the directory name to output fitting result.
Edited by Zhou, Wenduo