Add Getter/Setter for DataTypeMap fields

[jdye64]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
Once a DataTypeMap is instantiated there is currently no way to access its fields from a 3rd party library.

Describe the solution you'd like
Add pub to DataTypeMap field members and also #[pyo3(get, set)] for Python access.

Describe alternatives you've considered
None

Additional context
None