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Rather than just crashing ...
This typically happens when there is a discrepancy between
some installed lib version and a required version within
a dependancy.

This should be a one-shot operation.

Since straight out of the db we only get an offset,
and the utc timezone is quite implicit. We want it to be explicit
since pandas can change it interpretation from version to version
(e.g. .23 to .24 in a .loc in the presence of a datetime with offset
but no tz).

(we know it is utc because:

 * postgres is configured (see conftest) to UTC
 * we insert as utc
)

It will help to seamlessly assemble queries out of
* plain string
* query parameters
* pre-built `sqlp` string + parameters elements

`sqlp` carries an sql string fragment plus its needed parameters

... rather than just a name list.

Since supervision uses one, we'd better do this right.
Was lots in the transition from sqlalchemy to plain sql.

We still keep the stripped changesets in db because it is basically
impossible to remove them using the current schema.

The foreign keys from series tables to the changeset table
turn it into a neverending operation.

Fix for this will belong to #54.

This highlight an issue: we don't have the name of the series of the
stripped changesets.

These orphans are created when strip is invoked.
Next, strip will use it.

Little has changed yet.

We have:
* TimeSerie -> timeseries
* .get_history -> .history
* .get_delta  -> .staircase

closes #47

Having a slightly cluttered changeset log is a much less serious issue
than having slow and/or failing delete operations.

We now mark the changeset entries to tell them from normal entries.

Proper handling of the changeset entries deletion will be addressed in
#54.

closes #51

The time is also proportional to the number of insertions.
With only one series one can measure .1 seconds,
with four (and still one deleted) one measures .17 seconds.

Also `get_delta` becomes `staircase`
and `get_history` becomes `history`

While doing so we also get rid of
* old migrations
* the rename utility

Hence we can drop the _testing hack and end users
will feel less punished.

And now have a test coverage for this.