Consolidate LSST schemas and enhance Alert for Pitt-Google-Broker/pull/263

[troyraen]

Closes #72.

• Consolidate LSST schemas into one schema named is "lsst". The user no longer supplies the schema version, it is now retrieved from the Avro header when an Alert is initialized. Fixes Refactor the LSST schema helper so the user does not need to know the schema version #72.
• Enhance Alert for Update ps_to_storage source code Pitt-Google-Broker#263.
  • Add properties name_in_bucket, ra, dec.
  • Add schema.version to attributes.
• Change default schema name "default_schema" -> "default" and add a schema map for it.
• Unit tests
  • Add tests for LSST schema and Alert properties.
  • Add randomly generated data for schema "lsst.v7_4.alert".

[codacy-production]

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Coverage variation	Diff coverage
✅ +2.01%	✅ 96.25%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (7c91733)	829	536	64.66%
Head commit (bda9398)	858 (+29)	572 (+36)	66.67% (+2.01%)

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Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#81)	80	77	96.25%

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[troyraen]

@hernandezc1 I'm going to go ahead and merge this because I want to do a more major refactor of the schemas in a fresh PR that is based on these changes. I could leave this open and still do that, but I don't want to make any more changes in this PR anyway because of the high potential for conflicts. If this PR needs changes, I'll make them in a followup.