pip3 install pyreduce-astro doesn't install required packages emcee and corner

[regislachaume]

A pip3 install pyreduce-astro inside a brand new virtual environment doesn't install the following requirements emcee & corner, so that examples (e.g. %run uves_example.py within ipython3) fail to execute if these are not installed manually. These packages are listed in requirements.txt but not in setup.py.

I would suggest to have the setup.py read the requirements.txt file in order to avoid double maintenance.