Add muparser to the installation guide

Summary

A new user would not be able to configurate DORiE right now without noticing that there is a missing package to install into the installation instructions.

#ubuntu

        apt update
        apt install cmake doxygen gcc g++ gfortran \
                    git libatlas-base-dev libfftw3-dev libfftw3-mpi-dev \
                    libfreetype6-dev libhdf5-mpi-dev libopenmpi-dev libpng-dev \
                    libsuperlu-dev libyaml-cpp-dev libxft-dev \
                    python3-dev python3-pip

#macOS
        brew update
        brew install cmake doxygen gcc libpng open-mpi \
                     pkg-config python3 superlu yaml-cpp
        brew install hdf5 --with-mpi
        brew install fftw --with-mpi

Steps to reproduce

Install DORIE following instructions in README.md.

What is the expected correct behaviour?

The user can install DORiE following our instructions.

Relevant logs, screenshots, files...?

...

Ideas how to fix this?

(Add them here, if you have any.)