add install target for python bindings

[EiffL]

At the moment, even when they are created, the python bindings are not correctly installed by CMake. This issue is to improve python support for nicaea

[EiffL]

@martinkilbinger Any interest in making nicaea a pip installable package ?

[martinkilbinger]

that'll be great! Is it easy to do?

[sfarrens]

It is, I can show if you like.

[cgiocoli]

Hi all, is there a way to do pip install with the new version of ncaea?