Band views of SubArrays of BandedMatrices

[jishnub]

This PR uses the fact that views of a BandedMatrix with AbstractUnitRange ranges have bands that are contiguous views of the original. This helps speed up indexing operations by forwarding these to the bandeddata.

On master

julia> A = brand(200, 200, 3, 4);

julia> B = view(A, 1:100, 1:100);

julia> @btime $B[band(0)];
  1.050 μs (1 allocation: 896 bytes)

This PR

julia> @btime $B[band(0)];
  214.281 ns (1 allocation: 896 bytes)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4f24c9d) 89.51% compared to head (cbb6f32) 89.55%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #405      +/-   ##
==========================================
+ Coverage   89.51%   89.55%   +0.03%     
==========================================
  Files          25       25              
  Lines        3529     3542      +13     
==========================================
+ Hits         3159     3172      +13     
  Misses        370      370              

Files	Coverage Δ
src/banded/BandedMatrix.jl	95.24% <100.00%> (+0.13%)	⬆️

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