Use Enums for restricted-input keyword arguments

[chris-maclean]

Many functions have keyword arguments that must be one of a few certain values. Currently PyWormbase is just putting the valid arguments in a list and verifying that this list contains the input. I can do better by putting the valid values into an enum and have the user pass an Enum value as the argument.

I can also have the function try to convert the kwarg to the appropriate Enum class if the arg is a string. That way, currently-working code won't break.