Add delete_point for morphology branch.

[drodarie]

As a user I want to be able to remove a point from a morphology branch. Added function delete_point in Branch. Fix walk.
Add tests and update documentation.

[codecov[bot]]

Codecov Report

Merging #751 (a8f7e39) into main (577f060) will decrease coverage by 0.04%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #751      +/-   ##
==========================================
- Coverage   74.37%   74.34%   -0.04%     
==========================================
  Files         134      134              
  Lines       15764    15741      -23     
==========================================
- Hits        11725    11703      -22     
+ Misses       4039     4038       -1     

Files	Coverage Δ
bsb/morphologies/__init__.py	81.27% <100.00%> (+0.19%)	:arrow_up:
tests/test_morphologies.py	99.86% <100.00%> (+<0.01%)	:arrow_up:

... and 13 files with indirect coverage changes