NCLX color profile rework

[bigcat88]

Fixes #64 .

Changes proposed in this pull request:

• Adds support for read nclx values
• Fixes memory leak
• NCLX write is still partial(it will be necoded in image itself, and libheif can't read that afterwards) due to libheif restrictions

[codecov]

Codecov Report

Merging #66 (1388fd0) into master (e865a21) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #66   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines         1077      1088   +11     
=========================================
+ Hits          1077      1088   +11     

Impacted Files	Coverage Δ
pillow_heif/private.py	100.00% <100.00%> (ø)