Define dataids for a BandedMatrix

[jishnub]

Profiling shows that this saves some time in matrix-vector multiplications.

julia> B = brand(100, 100, 0, 0);

julia> v = rand(size(B,2));

julia> w = similar(v, size(B,1));

julia> @btime mul!($w, $B, $v);
  763.242 ns (0 allocations: 0 bytes) # master
  698.709 ns (0 allocations: 0 bytes) # This PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.74%. Comparing base (c4e85f4) to head (74c71c2).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #472      +/-   ##
==========================================
+ Coverage   89.71%   89.74%   +0.02%     
==========================================
  Files          25       25              
  Lines        3685     3694       +9     
==========================================
+ Hits         3306     3315       +9     
  Misses        379      379              

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