Migration/name change

[danjgale]

As mentioned in #24, I think niimasker should become nixtract (short for NeuroImaging eXtract) to reflect it's expanding uses with not only volumes, but surfaces. This would involve migrating to a new repository once it is ready.

Timing-wise, this should be done after #20, as it is already in progress.

The migration should then occur and include the following updates upon release:

• Conversion to pydra (Use Pydra to manage dataflow #21) if this works nicely
• BIDS formatting (BIDS? #23)
• Surface extraction availble (Surfaces #24)
• A proper Read the Docs site for documentation and examples.

So, nixtract is essentially niimasker 2.0 that is very different from the original. In this repository, we can switch the readme to a have a big 'no longer maintained, this has been moved to nixtract' message at the top.

[danjgale]

This has been dealt with now; everything is now at: https://github.com/danjgale/nixtract