Fixed naming bug in TuflowModel.missing_model_files.
Fixed naming bug in TuflowModel.missing_model_files.

TuflowLoader prepended an underscore to the name. This has
now been changed to add missing files directly to the
missing_model_files list in the TuflowModel instance held
by the loader rather than setting it after the load.

Tidied up some unecessary old code and comments in the loader
as well.

duncan-r/SHIP