Extension of COESA model to 86km - 2nd try

Hello  Juan,

Here is a new pull request for the extension of the COESA model. 

I got rid of the unnecessary `np.array()` by changing them for `np.asarray()` which is indeed much faster.
I used `np.exp` rather than `scipy.exp` thus avoiding an import.
I PEP8-cleaned `coesa.py` even though it is not perfect yet.
I did not create a constant module since most of the constants are either very specific to coesa or already available in the `scipy.constants` module. It's up to you if you want to create such a module, maybe it can be useful when this scikit grows.

I think this is better now. Please, do not hesitate if you have further remarks.

Finally, I followed the steps you kindly suggested to me regarding git. Everything went all right as you can see. Thank you.

Regards.

Gilles