Merge branch 'tisaac/feature-yaml-string' into 'master'

Added PetscOptionsInsertStringYAML and -options_string_yaml

See merge request petsc/petsc!2791

Added PetscOptionsInsertStringYAML and -options_string_yaml

In order to test this new feature, I had to add the ability to parse strings
with newlines in the 'args:' of tests.