only import pandas when `export_pandas` gets called

[dakra]

I guess in most production and testing environments pandas is not installed, so every time you import pydruid anything you get an (annoying) warning message printed even though you never use the query.export_pandas function.

This pull moves the import into the function so you only get an error when you actually call export_pandas.

[nishantmonu51]

:+1:

[erikdubbelboer]

Is there any plan om merging this soon? Seems like a no-brainer to me.

[fjy]

👍