Make BandedMatrix immutable

[dlfivefifty]

No description provided.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (acebbf1) 89.50% compared to head (43d4ddf) 89.51%.
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #403      +/-   ##
==========================================
+ Coverage   89.50%   89.51%   +0.01%     
==========================================
  Files          25       25              
  Lines        3525     3529       +4     
==========================================
+ Hits         3155     3159       +4     
  Misses        370      370              

Files	Coverage Δ
src/BandedMatrices.jl	100.00% <ø> (ø)
src/banded/BandedMatrix.jl	95.11% <ø> (ø)

... and 1 file with indirect coverage changes

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