Add imsobel() and imfilter() (lepto) functions.

[joa-quim]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 78.78788% with 7 lines in your changes missing coverage. Please review.

Project coverage is 85.96%. Comparing base (beff732) to head (8e7f64f).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/libleptonica.jl	40.00%	6 Missing ⚠️
src/lepto_funs.jl	95.23%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1648      +/-   ##
==========================================
- Coverage   86.02%   85.96%   -0.07%     
==========================================
  Files         151      151              
  Lines       21190    21222      +32     
==========================================
+ Hits        18229    18243      +14     
- Misses       2961     2979      +18     

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