Fix Binder instance run

In MR !62 (merged); I broke the generation of Binder instance, as strangely enough the Conda environment that works on Linux and for Docker images utterly fail in Binder (and the version that works for Binder fails locally and for Linux images... )

Here I fix this, albeit not in the prettiest possible way (basically I provide two different Yaml environments: one for local/Docker and another for Binder.

I have updated the README to reflect both changes.

Note: I have in fact locally worked on top of the branch provided in !62 (merged) and have cherry-picked the commits to put them on top of current master branch.

docker/Dockerfile.xeus-cling

@@ -10,9 +10,9 @@ RUN chown ${USER}:${USER} /opt/conda
 USER ${USER}
 
 # Create the environment:
-WORKDIR /home/${USER}/gettingstartedwithmoderncpp
-COPY environment.yml .
-RUN conda env create -f environment.yml 
+WORKDIR /home/${USER}/training_cpp
+COPY environment_local.yml .
+RUN conda env create -f environment_local.yml 
 
 # Make RUN commands use the new environment:
 SHELL ["/bin/bash", "-c"]