Soft dependency on Goalie to avoid duplication

[jwallwork23]

It does make me wonder, though, whether we should avoid the duplication so this doesn't happen again. We obviously don't want Thetis to depend on Goalie, but maybe we could put something in here so that this part of the code only works if you have Goalie installed and if so imports form2indicator?

Originally posted by @jwallwork23 in #349 (review)