Add type hints to Timeseries Factory, add custom literal types

Suggestion for type hinting in Timeseries Factory.

Biggest change: Parameters interval and type are hinted with custom literals

Literals are specified in geomag_types.py:

DataInterval = Literal["tenhertz", "second", ...]

DataType = Literal["adjusted", ...]

My thinking is that the geomag_types module holds custom types we would like specified more than their underlying data type. @jmfee thoughts?