Update compilation instructions

[speth]

While Ember compiles fine using the current Cantera release (2.5.1), the instructions in installation.rst are now behind by several Cantera versions, as well as still referencing Python 2.7. This should probably be updated in conjunction with a new tagged Ember version.

[marina8888]

Hi @speth. I was wondering if there were still plans for this? This is a code I would be so interested in using and I'm not sure there are any alternative codes available that cover these types of flamelets at present.

As an inexperienced user, I am having difficulty compiling on macos and even attempting the install on windows, pip has dropped support for python2.7, meaning manually selecting/downgrading dependent library versions just to try to get to pip2 to install for cython etc...

Alternatively, it might be possible to distribute this with pypi by taking just the C/C++ files as external directory and rewriting the top user functions with cffi... I'm not too familiar with the repo - please could it be possible to have your opinion on this?