Handling decimal types explicitly

[randyzwitch]

Related #191

From what I can see, we don't explicitly handle the Python decimal type in our create table statements. Evaluate, then if not, implement this so that users passing explicit decimal types get the proper upload behavior into OmniSci

https://docs.python.org/3.7/library/decimal.html

[randyzwitch]

Update: filed internally as https://jira.omnisci.com/browse/BE-3467 for decimal downcast