Fix for Graph.describe() when the graph has a string index (#759)

[u3ks]

This PR fixes #759 .

Pandas aggregation functions pass the original string index to numba, and thats why it fails the type check.
To fix this we can either have two new numba mode calculation functions - one for strings, one for numbers or we can explicitly recast the index to integers, carry out the calculations using the numeric index and then reindex the results before returning them.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.4%. Comparing base (27776c3) to head (5f0c0eb).
Report is 36 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #760     +/-   ##
=======================================
- Coverage   85.4%   85.4%   -0.0%     
=======================================
  Files        150     150             
  Lines      15975   15983      +8     
=======================================
+ Hits       13641   13644      +3     
- Misses      2334    2339      +5     

Files	Coverage Δ
libpysal/graph/base.py	96.5% <100.0%> (+<0.1%)	⬆️
libpysal/graph/tests/test_base.py	100.0% <100.0%> (ø)

... and 2 files with indirect coverage changes