DiscretizedScale: Move to proper place, fix a minor problem

[janezd]

Issue

DiscretizedScale was defined in module owscatterplotgraph. It belongs to plotutils.

When values were ints that didn't fit into 64 bits, numpy stored thresholds as instances of object instead of float. This is now fixed by explicit conversion to float.

Includes

• [X] Code changes

[codecov[bot]]

Codecov Report

Merging #6055 (eda5528) into master (b1aa139) will increase coverage by 0.01%. The diff coverage is 95.52%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6055      +/-   ##
==========================================
+ Coverage   86.46%   86.47%   +0.01%     
==========================================
  Files         315      315              
  Lines       67600    67614      +14     
==========================================
+ Hits        58448    58467      +19     
+ Misses       9152     9147       -5