prepost.hawcstab2: more robust induction file reading, add tests

wetb/prepost/h2_vs_hs2.py

wetb/prepost/hawcstab2.py

wetb/prepost/tests/data/demo_gendlc/DLCs.xlsx

wetb/prepost/tests/data/demo_gendlc/DLCs/DLC12.xlsx

wetb/prepost/tests/data/demo_gendlc/DLCs/DLC13.xlsx

wetb/prepost/tests/data/dtu10mw_nofull_defl_u10000.ind

wetb/prepost/tests/data/dtu10mw_nofull_fext_u10000.ind

wetb/prepost/tests/data/dtu10mw_nofull_u10000.ind

wetb/prepost/tests/data/dtu10mw_nogradient_defl_u10000.ind

wetb/prepost/tests/data/dtu10mw_nogradient_fext_u10000.ind

wetb/prepost/tests/data/dtu10mw_nogradient_u10000.ind

wetb/prepost/tests/data/dtu10mw_v1_defl_u10000.ind

wetb/prepost/tests/data/dtu10mw_v1_fext_u10000.ind

wetb/prepost/tests/data/dtu10mw_v1_u10000.ind

wetb/prepost/tests/test_hawcstab2.py