Switch from apt-get to pip for python packages

Install numpy, scipy and matplotlib via the pip3 command and not via ubuntu's
package manager, since this would install older versions.

README.md

@@ -21,10 +21,17 @@ additional packages `numpy`, `scipy` and `matplotlib` are useful for data
 analysis and data presentation. To install all the packages on Ubuntu, you
 can run the following command line.
 
-
 ```bash
-sudo apt-get install python3 python3-numpy python3-scipy python3-matplotlib
+sudo apt-get install python3 python3-pip
+sudo pip3 install numpy scipy matplotlib
 ```
+
+This installs the most recent versions these three packages. If you install the
+packages via ubuntus package manager, you will get older version. Ubuntu's
+packages can hide the more up-to-date ones installed by pip3. My personal
+recommandations is to always use pip and never install python packages via
+ubuntus package manager if you want to use recent version of the packages.
+
 <!--
 Doxec in a docker container needs a slightly different command, please keep them
 in-sync.
@@ -32,7 +39,8 @@ in-sync.
 <!-- console
 ```bash
 $ apt-get update
-$ apt-get install -y python3 python3-numpy python3-scipy python3-matplotlib
+$ apt-get -y install python3 python3-pip
+$ pip3 install numpy scipy matplotlib
 ```
 -->