feat(emx2-pyclient): make keep_default_na a parameter in "get"

[dtroelofsprins]

Is your feature request related to a problem? Please describe.
When retrieving data using the get method from the emx2-pyclient, the API response is transformed into a dataFrame using pd.read_csv with keep_default_na = False. It turns out that if a column contains missing value all cells become type=str. Which might in some cases result in unexpected results.

Describe the solution you'd like
Make the keep_default_na optional (so include it as a parameter) with default option = False for backwards compatibility.

Describe alternatives you've considered

• rewrite data handling to take into account that integers and bools are strings

Additional context