- remove repeated CMAKE_BUILD_TYPE
- bump lcov to 1.15
- use new --exclude flag to skip generated sources

With gcc 8 as a compiler + a new version of lcov (#1350) codecov started tracking generated source files and coverage dropped to < 60%. Not sure if this is a bug in lcov or not. I've added these new flags for lcov: `--exclude "*/ext/*" --exclude "*/test/*" --exclude "*/mechanisms/*"` to fix it.

* Make Gitlab CI push images to CSCS' JFrog registry reg.giuv.cscs.ch so that sarus can pull images on Daint again. 
  This URL is probably temporary until JFrog officially goes into production later this month.
  Unfortunately JFrog is currently behind the firewall.
* Upgrade g++ to g++8 on the Docker image.
* Only require libxml when compiling with NeuroML support.

Update all Python scripts used during build/testing/running Arbor to explicitly use Python 3.

This is copied from DLA Future. It should set the CI status on the commit from the PR instead of just on the Bors commit. This way the gitlab CI status also appears in the checks section below the PR.

This is a bit experimental. It builds arbor with `-fprofile-arcs -ftest-coverage` and creates a docker image with just the project binaries, lcov, gcov and generated `.gcno` files. This should still be reasonably small since there are no object files etc. When run on daint, `.gcna` files are produced (should be thread / process safe according to gcc's manual) inside of the container, and `lcov` is used to combine all those things into a single file with a random name in the mounted git project folder (hopefully this solves clashes with multiple nodes). After all tests are run, the combined reports are uploaded to codecov.io.

Combine badges on readme and move ci git submodule to `ext/`

* Add Dockerfiles for MPI + CUDA test builds on Daint
* Add bors.toml for CI on Daint
* Add the separate ci repo as submodule for convenience
* Add a CI badge for master to the top of the readme