Allow passing in references of type UUIDs or str besides model instances

[denizs]

At the moment, the Python SDK assumes a linear chain of SDK calls traversing from sites to appliances to variables to timeseries.
However, another use case involves instances where the use knows the appliance or variable uuid(s) beforehand, rendering calls to other SDK methods obsolete.

Thus, we want to enable users of the Python SDK to pass in references to appliances and variables of type UUID or str as well.