write_template() is basically a hand-rolled f"", with a few differences

• The substitution is ${val}, not {val}
• The substitution can't be an arbitrary python expression, but must be one of a set of values defined in a dicationary.

The only interesting thing this does that f"" doesn't is allow the dictionary to be persistently attached to the writer object through function calls. This is only used in one place, however, and the called function modifies the dictionary contents during its execution, so it's not exactly clean.

This functionality likely exists because IDL dates all the way back to python 2. In the name of maintainability, we should purge this logic in favor of more flexible f"" usage.