Convenience find_by methods

[BenGalewsky]

As a foundry dataset user I want to easily be able to find a dataset by DOI or by source_id so I can go directly to my dataset of interest

Description

Add find_by_doi and find_by_dataset to the Foundry class.

These methods return a single Dataset instance

Assumption

1. if the source ID or DOI don't exist, throw an informative exception

Acceptance Criteria

f = Foundry()
ds = f.find_by_doi("DOI12/344")

ds2 = f.find_by_source_id("1233423")

f.find_by_source_id("doesnotexist")
f.find_by_doi("doesnotexist")