Minor memory optimization for Zernike jacobi matrices

[TSGut]

Symmetric() means we don't actually have to store some zero bands when initially generating them.

[codecov]

Codecov Report

Merging #122 (c847428) into master (e065da9) will increase coverage by 0.26%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #122      +/-   ##
==========================================
+ Coverage   95.97%   96.23%   +0.26%     
==========================================
  Files           4        4              
  Lines         720      718       -2     
==========================================
  Hits          691      691              
+ Misses         29       27       -2     

Impacted Files	Coverage Δ
src/disk.jl	98.22% <100.00%> (+0.86%)	⬆️

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[TSGut]

@dlfivefifty Ready for merge.

[dlfivefifty]

(technically not a "memory optimization" as Zeros doesn't use any memory...)

[TSGut]

Interesting, I thought explicitly constructed zeros are still stored! In that case just a minor tidying of a redundancy. :)