Improve type-inference in blockbandwidths for BlockBandedMatrix

[jishnub]

Using map instead of broadcasting over _broadcast_blockbandwidths improves type-inference, as this is a much simpler operation. This should be safe, as bc.args is a Tuple, and the map would be equivalent to a broadcast.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.58%. Comparing base (7467a90) to head (06b5c14).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #210      +/-   ##
==========================================
+ Coverage   89.55%   89.58%   +0.02%     
==========================================
  Files          11       11              
  Lines        1111     1114       +3     
==========================================
+ Hits          995      998       +3     
  Misses        116      116              

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