Specialize unaliascopy for a BandedMatrix

[jishnub]

This might help with certain unaliasing operations, as unaliascopy requires the type of the matrix to be preserved.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.79%. Comparing base (c4e85f4) to head (c7fec58).
Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #478      +/-   ##
==========================================
+ Coverage   89.71%   89.79%   +0.08%     
==========================================
  Files          25       27       +2     
  Lines        3685     3715      +30     
==========================================
+ Hits         3306     3336      +30     
  Misses        379      379              

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