Add common morphological operations (erode, dilate, ope, close).

[joa-quim]

Make the pix2img and img2pix work also with 1 bpp images.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 77.48344% with 34 lines in your changes missing coverage. Please review.

Project coverage is 86.01%. Comparing base (2b4996d) to head (88c0fd2).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/lepto_funs.jl	83.84%	21 Missing ⚠️
src/libleptonica.jl	38.09%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1640      +/-   ##
==========================================
- Coverage   86.02%   86.01%   -0.02%     
==========================================
  Files         151      151              
  Lines       20919    21043     +124     
==========================================
+ Hits        17996    18100     +104     
- Misses       2923     2943      +20     

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