Add missing Animation constructor

[agdestein]

Description

This adds a constructor Animation(dir) in addition to the two existing ones Animation() and Animation(dir, frames).
The docstring already mentions Animation(dir) as an option (see #4568).

Attribution

• I am listed in .zenodo.json (see How to get acknowledged #3503)

Things to consider

• Does it work on log scales?
• Does it work in layouts?
• Does it work in recipes?
• Does it work with multiple series in one call?
• PR includes or updates tests?
• PR includes or updates documentation?

[BeastyBlacksmith]

Would you mind adding a test?

[agdestein]

If dir does not exist, adding a frame throws a SystemError (missing directory). We could alternatively create this directory in the constructor, e.g. ispath(dir) || mkpath(dir). This is also done for the empty constructor with mktempdir.

[BeastyBlacksmith]

I think the current behaviour is reasonable.