Create new granule_fill_order attribute to specify granule allocation preference

[craigthomas]

This PR updates the DiskFile class to have an attribute called granule_fill_order. This list-like object is exactly 68 elements long. Each entry in the list specifies the granule number that should be checked to see if it is empty. The order of the list reflects which granules are allocated first. In DiskConstants there is a constant called GRANULE_FILL_ORDER that mimics the way Disk BASIC attempts to allocate granules on the disk. In general, Disk BASIC attempts to fill granules near the center of the disk first to minimize head stepping. This same ordering is implemented within this PR. The class can be instantiated with a different ordering list, allowing for future customization when generating disk images. Unit tests updated to cover new conditions. This PR closes #73

[codecov]

Codecov Report

Merging #80 (ec34715) into main (97b8796) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #80      +/-   ##
==========================================
+ Coverage   94.43%   94.44%   +0.01%     
==========================================
  Files          15       15              
  Lines        1779     1783       +4     
==========================================
+ Hits         1680     1684       +4     
  Misses         99       99              

Impacted Files	Coverage Δ
cocoasm/virtualfiles/disk.py	93.60% <100.00%> (+0.10%)	⬆️

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