Support ZTF lite schema

[hernandezc1]

Related to #89

To Do:

• (Chris) Add a schema map for the ZTF lite schema.
• (Troy) Idea for organization: Add StetsonJ statistics to LSST science pipeline Pitt-Google-Broker#271 (comment).
• (Troy) Implement support for that schema map in Alert and other classes. Hopefully most methods will be able to handle the new map without any trouble but I'm guessing there will be a few that need some tweaking. The important methods to focus on are those that will be needed by the cloud run implementation of the ZTF SuperNNova module being developed in Use Cloud Run for ZTF's SuperNNova module Pitt-Google-Broker#265.
• (Troy) Try removing the "default" option for Alert.schema_name. I think it's redundant with None and thus just confusing.
• (Troy) Issues identified when altering alert content #97 is a more general issue (not only for ZTF alerts), but there's a lot of overlap with the above so it should probably be addressed at the same time.
• Make Alert.get raise a warning if a key is passed that is not one of our generic field names.

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[hernandezc1]

Pointing out that both:
class_name = schema_name[0].upper() + schema_name[1:] + "Schema" and

schema_name_parts = schema_name.split(".")
class_name = (
    schema_name_parts[0].capitalize()
    + "".join(part for part in schema_name_parts[1:])
    + "Schema"
)

will not work if schema_name = "elasticc.v0_9_1.brokerClassification" or schema_name = "elasticc.v0_9_1.alert".