Make create_indexes independent

[gwbischof]

Databroker.v0 and this suitcase now have conflicting indexes due to this PR: #34
Where we decided to make resource uid unique. (Databroker.v0 also has code in it to create indexes.) This causes an exception when the index is created, if a conflicting index already exists. Which means that you can use this suitcase and databroker.v0 on the same database until it is fixed.

@jklynch and I think that we should make index creation independent of suitcase and databroker.
@danielballan suggested that we make the index creation independant, but we leave it in this package, and don't call it in the suitcase.init

[danielballan]

@gwbischof summarized my view accurately above; to add a detail, tools like databroker-unpack will need access to this so they can seamlessly create/configure new databases, so it needs to stay in an importable package (as a opposed to be encoded in ansible).

A big +1 for separating this administrative activity from the reading and writing tools.

[danielballan]

To outline practical steps here, in suitcase/mongo_normalized/__init__.py:

1. Rename _create_indexes to create_indexes, a public method.
2. Do not call create_indexes in __init__.

[danielballan]

3. Add a line to https://blueskyproject.io/databroker/how-to/mongo-backed-catalog.html which comes from https://github.com/bluesky/databroker/blob/main/docs/source/how-to/mongo-backed-catalog.rst along the lines:

python -c 'import suitcase.mongo_normalized; suitcase.mongo_normalized.Serializer("mongodb://...").create_indexes()'