Use enum for input data type

[duckontheweb]

The fields for "The data type of the parameter" should be an enum of values like data_type in STACs raster extension. Otherwise you get a lot of different values for the same data types.

Originally raised in #25

[duckontheweb]

For reference, here is the list from the Raster Extension: https://github.com/stac-extensions/raster#data-types