Document requirements of Python examples

[speth]

The examples shown at https://cantera.org/examples/python/index.html do not list the required version of Cantera or other dependencies. This leads to problems when users try to run them with incompatible versions of Cantera (see #728). A standard method for documenting these requirements was established in #729, and implemented for the ic_engine.py example, adding the line:

Requires: Cantera >= 2.5.0, scipy >= 0.19, matplotlib >= 2.0

to the script's docstring.

Similar Requires: sections should be added to the other Python examples. To simplify determining the appropriate minimum versions, I would suggest:

• the only Cantera versions that need to be checked are 2.4.0 and 2.5.0
• Cantera dependencies such as NumPy do not need to be documented unless an especially new version is required
• The minimum version of other dependencies can be just the minimum tested version, rather than having to install very old versions just for the sake of testing

[bryanwweber]

I believe this was completed in #768