add mesa data set

[sqhou]

I just add a small documentation and a example for MESA data.

[fherwig]

1. Why do you plot log10(s.get('star_age') - why log? Does that make sense? Sometime people plot log of the remaining time. But that is not what this is. You could plot as a function of model number.

2. No - there is no direct way to extract units from MESA output. But I do believe there is somewhere, either on the mesa web page or in a MESA code paper a table. Maybe someone can find it and put a link into this notebook.

@sqhou can you fix these two items? (for 2 maybe just remove the question, or turn it into a comment?)

[sqhou]

Sorry, just show how to operate it, no specific physical meaning. OK, I have added the contents about unit of mesa data.

[fherwig]

@sqhou there is now a conflict - can you resolve it please? Someone has change the Star_explore notebook as well, and git can not automatically merger it

[sqhou]

I have done as the command line instructions. I wonder is it resolved?

[fherwig]

this has still unresolved conflict, maybe checkout Star_explore.ipynb on fresh branch, apply changes and make new pull request, and delete this pull request and branch

[fherwig]

This requires manual merge with latest version.