Changed argument leafs to leaves, with deprecation warning in ontograph

[francescalb]

Description:

The plural of leaf was written leafs in all arguments etc. Changed it to leaves.

Added a deprecation warning on the tool ontograph, but apart from that did not update with deprecation warnings after discussing the need with @jesper-friis.

Type of change:

• Bug fix.
• New feature.
• Documentation update.

Checklist:

This checklist can be used as a help for the reviewer.

• Is the code easy to read and understand?
• Are comments for humans to read, not computers to disregard?
• Does a new feature has an accompanying new test (in the CI or unit testing schemes)?
• Has the documentation been updated as necessary?
• Does this close the issue?
• Is the change limited to the issue?
• Are errors handled for all outcomes?
• Does the new feature provide new restrictions on dependencies, and if so is this documented?

Comments:

[codecov-commenter]

Codecov Report

Merging #564 (6d6b73d) into master (5d89725) will increase coverage by 0.14%.
The diff coverage is 82.08%.

❗ Current head 6d6b73d differs from pull request most recent head 366f1e8. Consider uploading reports for the commit 366f1e8 to get more accurate results

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@@            Coverage Diff             @@
##           master     #564      +/-   ##
==========================================
+ Coverage   65.85%   66.00%   +0.14%     
==========================================
  Files          16       16              
  Lines        3099     3112      +13     
==========================================
+ Hits         2041     2054      +13     
  Misses       1058     1058              

Impacted Files	Coverage Δ
ontopy/ontodoc.py	67.51% <64.28%> (ø)
ontopy/utils.py	62.33% <75.00%> (+0.08%)	⬆️
ontopy/ontology.py	70.27% <90.00%> (ø)
ontopy/__init__.py	80.00% <100.00%> (ø)
ontopy/graph.py	62.47% <100.00%> (+0.84%)	⬆️

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[CasperWA]

Excellent! :) You probably need to test the DeprecationWarning thing locally, once implemented (see comment below).

[CasperWA]

I guess the deprecation warning could be tested (and tested that it still works with leafs) but I don't consider it a must, to be honest.