various updates - improve test suite

[t-bltg]

• fix Bug: cgrad changes alphas (to illegal values above 1) #156;
• fix palette() with n=1 fails #155;
• add Makie to downstream tests (fix Increase coverage #150);
• enhance fallback ticks by using strict_span;

[codecov-commenter]

Codecov Report

Attention: 27 lines in your changes are missing coverage. Please review.

Comparison is base (b5539fb) 69.49% compared to head (210d960) 72.05%.

Files	Patch %	Lines
src/ticks.jl	48.78%	21 Missing ⚠️
src/adapted_grid.jl	66.66%	3 Missing ⚠️
src/colorschemes.jl	92.30%	2 Missing ⚠️
src/colors.jl	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #157      +/-   ##
==========================================
+ Coverage   69.49%   72.05%   +2.55%     
==========================================
  Files           7        7              
  Lines         659      662       +3     
==========================================
+ Hits          458      477      +19     
+ Misses        201      185      -16     

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