Add Literal types (aka move to Python 3.8)

[GilesStrong]

In a few locations, string values arguments are used in LUMIN and specific value are expected. Python 3.8 introduced literal types (https://realpython.com/python38-new-features/), where the set of expected values can be stated in the method definitions. This is potentially useful, but would then require setting the minimum version of python for LUMIN from 3.6 to 3.8, which could be disruptive to our user-base. (Although, since I doubt we actually have a consistent user-base, this change may not be disruptive and should be done sooner rather than later).