Add `InvertedRoI` mixin and `FixedIndegree` strategy

[Helveg]

Describe the work done

The InvertedRoI mixin lets users make strategies that look for the presynaptic RoI of a post chunk, rather than the postsynaptic RoI of a pre chunk.

The FixedIndegree strategy connects cell populations with a fixed number of random presynaptic targets for each postsynaptic target.

Tasks

• [x] Added tests
• [ ] Updated documentation

[codecov[bot]]

Codecov Report

Merging #729 (c65a330) into main (1cdfa8b) will increase coverage by 0.19%. The diff coverage is 98.61%.

@@            Coverage Diff             @@
##             main     #729      +/-   ##
==========================================
+ Coverage   74.03%   74.22%   +0.19%     
==========================================
  Files         134      134              
  Lines       15592    15663      +71     
==========================================
+ Hits        11543    11626      +83     
+ Misses       4049     4037      -12     

Impacted Files	Coverage Δ
bsb/mixins.py	93.61% <92.30%> (-0.51%)	:arrow_down:
bsb/connectivity/general.py	71.71% <100.00%> (+11.71%)	:arrow_up:
bsb/connectivity/strategy.py	86.90% <100.00%> (+0.48%)	:arrow_up:
tests/test_connectivity.py	93.61% <100.00%> (+0.49%)	:arrow_up:

... and 2 files with indirect coverage changes