Implement function to list data files

[CyberDrudge]

Fixes #586

Function cantera.data.files() returns list of all data files. It can also be used to get files of a certain type by specifying the required extension.

[bryanwweber]

Thanks for sending this change! A few changes to request. When you make them, please squash all of your changes into one commit and force push to this branch to update the pull request.

[bryanwweber]

@CyberDrudge Thank you for the quick fixes! Can you squash the three commits here into one commit and force-push to this branch? If you need help with that, just let me know 😄 Thanks!

[CyberDrudge]

@bryanwweber no worries, a bit of googling showed me the way.

[bryanwweber]

@speth The function here is named files. I'm not totally sure that's the best name for this API, I wonder if list_files or list_data_files or print_data_files or something else would be better?

@CyberDrudge Can you please add documentation to this function? It would look something like

def files(ext=''):
    """
    Overall description of what this function does.

    :param ext:
        Description of what the ext argument does
    """

Thanks!

[CyberDrudge]

Changed function name and added Documentation. I will edit name and squash changes if required.

[CyberDrudge]

@bryanwweber does this still require any updates before it gets merged?