added model decisions for choosing backweuler and ida

build/source/dshare/get_ixname.f90

build/source/dshare/var_lookup.f90

build/source/engine/mDecisions.f90

build/source/engine/opSplittin.f90

...h_tests/celia1990/settings/summa_zDecisions_celia1990.txt

...sts/colbeck1976/settings/summa_zDecisions_colbeck1976.txt

...tests/miller1998/settings/summa_zDecisions_miller1998.txt

...sts/mizoguchi1990/settings/summa_zDecisions_mizoguchi.txt

...borght2005/settings/summa_zDecisions_vanderborght2005.txt

utils/laugh_tests/wigmosta1999/settings/summa_zDecisions.txt