extending -otsu with parameters to add multiple threshold functionality , plus a lot of small fixes on documentation and comments

[br-cpvc]

Hi Paul. Thanks for the positive feedback and the suggestions in PR https://github.com/pyushkevich/c3d/pull/23

I have redone the branch in PR https://github.com/pyushkevich/c3d/pull/23 with the following changes:

• renamed the get_int_args function to ReadIntegerArgs as suggested
• removed the '-label-voting' patch, so this can live in a separate branch/PR

[pyushkevich]

Really appreciate the contributions!