Move NLExtract Stetl Components to Stetl repo

**Is your feature request related to a problem? Please describe.**
The NLExtract [gfspreparationfilter](https://github.com/nlextract/NLExtract/blob/master/stetlcomponents/gfspreparationfilter.py) and [subfeaturehandler](https://github.com/nlextract/NLExtract/blob/master/bgt/etl/stetlbgt/subfeaturehandler.py) 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 #102 is in
* progress NLExtract fully to Python3, see [this issue](https://github.com/nlextract/NLExtract/issues/265)
* add [gfspreparationfilter](https://github.com/nlextract/NLExtract/blob/master/stetlcomponents/gfspreparationfilter.py) and [subfeaturehandler](https://github.com/nlextract/NLExtract/blob/master/bgt/etl/stetlbgt/subfeaturehandler.py) to Stetl GH `Filters`
* NB the latest/fixed `subfeaturehandler` is in [1.4.0 NLExtract branch](https://github.com/nlextract/NLExtract/blob/1.4.0/bgt/etl/stetlbgt/subfeaturehandler.py)
* 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.