Move NLExtract Stetl Components to Stetl repo

[justb4]

Is your feature request related to a problem? Please describe.
The NLExtract gfspreparationfilter and subfeaturehandler are generic enough to be moved to Stetl. This will ease running Stetl-based NLExtract configs, especially with Docker.

Describe the solution you'd like
These steps in this order should be performed:

• progress Stetl to v2.1, at least that PR WIP #99 and #93 - various version upgrades mainly Docker #102 is in
• progress NLExtract fully to Python3, see this issue
• add gfspreparationfilter and subfeaturehandler to Stetl GH Filters
• NB the latest/fixed subfeaturehandler is in 1.4.0 NLExtract branch
• some Python module naming changes
• add unit tests
• remove these two from the NLExtract GitHub
• adapt NLExtract Stetl configs for BGT and BRK and others for these changes
• test, test

Describe alternatives you've considered
Not really.

Additional context
N.A.