Use MulAdd's CZeros to remove duplicate zero-fill

[jishnub]

With this, multiplying a BandedMatrix by a OneElementVector would be as performant as slicing.

julia> B = brand(6000,6000,4000,4000); O = OneElement(1, 3, size(B,2));

julia> @btime $B * $O;
  3.826 μs (2 allocations: 46.92 KiB)

julia> @btime $B[:,$O.ind...];
  4.013 μs (4 allocations: 47.00 KiB)

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (ff75189) 89.55% compared to head (213a970) 89.59%.
Report is 2 commits behind head on master.

Files	Patch %	Lines
src/banded/gbmm.jl	89.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #424      +/-   ##
==========================================
+ Coverage   89.55%   89.59%   +0.03%     
==========================================
  Files          25       25              
  Lines        3544     3557      +13     
==========================================
+ Hits         3174     3187      +13     
  Misses        370      370              

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