Implement `FoundryDataset.to_pandas()`

[ascourtas]

As a user, I want to load a dataset as a pandas dataframe.

Assumptions:

A user finds the dataset they want by running this code:

f = Foundry()
datasets = f.search("DOI.123/445")

assert len(datasets) == 1
res = datasets[0].get_as_dict()

Instead of using get_as_dict(), the user wants to get the dataset as a pandas dataframe. Create a method get_as_pandas to allow the user to download the data in this format.

Acceptance Criteria

Given I know what dataset I want to use, when I specify that I want it in a pandas format, then the data downloads in the format I specified.