Extension of COESA model to 86km + minor changes

Hello,

I added to your COESA model the ability to compute atmosphere parameters up to 86 km (it was limited to 11 km before) consistently with the 1976 U.S. standard atmosphere. 

I tried to add some functionality (for instance it is now possible to choose if the altitude shall be passed in geometric of in geoppotential, the last beeing default) remaining consistent with what was before.

It is not the best code ever but it seems to work. It passes the tests. I did not add test for altitude higher than 11km because the values in si2py.prt for altitudes higher than 20000 m seem weird to me. Indeed, temperature seems to remain constant (216.7 K) even at 20400 m, but if you look at the original U.S. standard atmosphere 1976 publication, temperature starts to grow for altitude higher than 20000 m (the slope is 0.001 K/m). It does not seem to be a mix-up between geopotential and geometric altitude too. There is probably something I don't catch.

Let me know if you think this pull request is OK or if it is not. Do not hesitate if you need more details, or something.

Best regards

Gillu