Changes to PolyChord install required for Docker files

This is just to note that the way that PyPolyChord is being installed in the Docker files no longer works, so if the Docker images get updated then the following changes will be required:

RUN git clone https://github.com/PolyChord/PolyChordLite.git \
&& (cd PolyChordLite && make pypolychord  MPI= && python setup.py install)

should change to

RUN git clone https://github.com/PolyChord/PolyChordLite.git \
&& (cd PolyChordLite && python setup.py --no-mpi install)

On a related note (with my own Docker image based off of the miniconda3 one). apt-get was running into problems not finding certain packages (as described [here]), so again if the Docker image gets updated at any time soon the following line might be required:

RUN sed -i "s#deb http://security.debian.org/debian-security stretch/updates main#deb http://deb.debian.org/debian-security  stretch/updates main#g" /etc/apt/sources.list

before running RUN apt-get update.