changes for fafbseg-py v3

[jefferis]

• new way to set up default datastack
• new function names for L2 skels/dotprops
• see #193

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (9c7315c) 54.33% compared to head (8804ac4) 54.38%. Report is 2 commits behind head on master.

Files	Patch %	Lines
R/fafbseg-py.R	87.50%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #194 +/- ## ========================================== + Coverage 54.33% 54.38% +0.04% ========================================== Files 38 38 Lines 4765 4770 +5 ========================================== + Hits 2589 2594 +5 Misses 2176 2176 ```

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