New test fixutre local_pacta()

[maurolepore]

This PR provides the new local_pacta() -- a test fixutre like many others in the withr package.

Also:

• The messy file structure of PACTA moved to inst/extdata/context so the root is a clean, small package that only wraps calls to docker.

• The main call to docker now happens in the package. We could expose arguments for the user to specify the location of input/ output/ and pacta-data/ on the host, to specify image:tag, command to run, etc.

• With this interface docker-compose could be not a system requirement for user but instead a convenience for developers.

[jdhoffa]

I have comments about this (which I think are understood, but want to make it explicit for posterity):

• The goal of pactaCore is exactly to clean up the messy file structure of PACTA, not to hide it
• Extracting it into inst/ is OK as an interim step in the refactoring process, but the goal should be to migrate the code back to the root of the directory
• In general, I am very particular that the Dockerfile should exist in the root directory (or should not exist at all), and that calls to R functions that spin up the docker image are OK again as an interim step, but we must prioritize removing these soon, as this is a very confusing structure that is going to make maintenance difficult. I understand that a Dockerfile in the root is not expected in a standard package structure and would need to be added to .Rbuildignore, but this is because Docker and packages aim to solve the same problem. I do think the Dockerfile still has value.

With all of that said, #49 is the obvious reason to add all of this scaffolding, and having this snapshot test structure will allow us (hopefully) to quickly develop/ refactor and extract the core code back out of inst.

All of that to say, I back this, but with many caveats, and I think it is important we deal with it quickly.