Document how to use convert with custom function

[javidy]

Fixes #542

This PR has the objective of...giving an example of using a custom function with petl.transform.conversions.convert()

Changes

1. Improved the docs about...petl.transform.conversions.convert() providing example usage of custom function

Checklist

Checklist for for pull requests including new code and/or changes to existing code...

• [ ] Code
  • [ ] Includes unit tests
  • [ ] New functions have docstrings with examples that can be run with doctest
  • [ ] New functions are included in API docs
  • [ ] Docstrings include notes for any changes to API or behaviour
  • [ ] All changes documented in docs/changes.rst
• [ ] Testing
  • [ ] (Optional) Tested local against remote servers
  • [x] Travis CI passes (unit tests run under Linux)
  • [x] AppVeyor CI passes (unit tests run under Windows)
  • [x] Unit test coverage has not decreased (see Coveralls)
• [ ] Changes
  • [ ] (Optional) Just a proof of concept
  • [ ] (Optional) Work in progress
  • [x] Ready to review
  • [ ] Ready to merge

[coveralls]

Coverage increased (+0.09%) to 90.007% when pulling 66f0716be94ae787fb30c8c3da95135e58c54fbf on javidy:patch-1 into e9b3e87f921f8310f058de905953c3cdd65c2b57 on petl-developers:master.

[javidy]

hi @juarezr , would you be kind to review my PR :)

[javidy]

thanks a lot @juarezr , it was my first PR and I'm happy that it was merged :) hope I'll have many more valuable contributions in future.

[juarezr]

Welcome @javidy and @carlcorder !!!

:1st_place_medal:

[juarezr]

It looks like docs building / deploying CI is broken now...

[javidy]

@juarezr how can I help to fix the issue? Would be happy to do that.

[javidy]

I'm running doctests using tox as mentioned in documentation. Maybe it'll give me a clue.

[juarezr]

Local doctests and CI are working fine.

The problem could be someplace involving:

• Github
• Travis CI
• readthedocs.io