Gemini GMOS telluric test

[kbwestfall]

This PR pairs with https://github.com/pypeit/PypeIt/pull/1209 .

It also adds a test of pypeit_tellfit for existing Gemini GMOS data that specifically uses pypeit.core.telluric.poly_telluric. I changed the way the telluric tests are executed, requiring that a "telluric parameter file" be provided. This slightly changes the execution of the Gemini GNIRS test, but the result is the same.

Here is the result of the new gemini_gmos/GS_HAM_R400_700 run:

% OMP_NUM_THREADS=1 ./pypeit_test develop -i gemini_gmos -s GS_HAM_R400_700 -r test_gmos.log
Running tests on the following instruments:
    gemini_gmos

Reducing data from gemini_gmos for the following setups:
    GS_HAM_R400_700

 0 passed/ 0 failed/ 0 skipped STARTED gemini_gmos/GS_HAM_R400_700 pypeit (without masters)
 1 passed/ 0 failed/ 0 skipped PASSED  gemini_gmos/GS_HAM_R400_700 pypeit (without masters)
 1 passed/ 0 failed/ 0 skipped STARTED gemini_gmos/GS_HAM_R400_700 pypeit_sensfunc
 2 passed/ 0 failed/ 0 skipped PASSED  gemini_gmos/GS_HAM_R400_700 pypeit_sensfunc
 2 passed/ 0 failed/ 0 skipped STARTED gemini_gmos/GS_HAM_R400_700 pypeit_flux
 3 passed/ 0 failed/ 0 skipped PASSED  gemini_gmos/GS_HAM_R400_700 pypeit_flux
 3 passed/ 0 failed/ 0 skipped STARTED gemini_gmos/GS_HAM_R400_700 pypeit_coadd_1dspec
 4 passed/ 0 failed/ 0 skipped PASSED  gemini_gmos/GS_HAM_R400_700 pypeit_coadd_1dspec
 4 passed/ 0 failed/ 0 skipped STARTED gemini_gmos/GS_HAM_R400_700 pypeit_tellfit
 5 passed/ 0 failed/ 0 skipped PASSED  gemini_gmos/GS_HAM_R400_700 pypeit_tellfit

--- PYPEIT DEVELOPMENT SUITE PASSED 5/5 TESTS (Masters ignored) ---
Testing Started at 2021-06-23T16:30:51.597857
Testing Completed at 2021-06-23T16:47:05.261199
Total Time: 0:16:13.663342