Fix warning

[nwlandry]

Added a more intelligible warning for sparse adjacency matrix. Fixes #302.

[codecov]

Codecov Report

Patch coverage: 98.59% and project coverage change: +3.86 🎉

Comparison is base (7d5accb) 86.39% compared to head (cb2866d) 90.26%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #315      +/-   ##
==========================================
+ Coverage   86.39%   90.26%   +3.86%     
==========================================
  Files          35       35              
  Lines        2757     2814      +57     
==========================================
+ Hits         2382     2540     +158     
+ Misses        375      274     -101     

Impacted Files	Coverage Δ
xgi/linalg/matrix.py	98.72% <95.00%> (+4.48%)	⬆️
xgi/classes/reportviews.py	92.07% <100.00%> (ø)
xgi/drawing/draw.py	69.54% <100.00%> (+45.52%)	⬆️

... and 2 files with indirect coverage changes

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[maximelucas]

Looks good to me, thanks.