Move docker images to central registry

[Peter9192]

Currently each ewatercycle.model.* has a _get_docker_image and _get_singularity_image. The images are coupled to the available_versions class property.

Ideally the system administrator would pull/build all images so the user can just start them. If they are not available, this leads to issues (eWaterCycle/grpc4bmi#100 and eWaterCycle/grpc4bmi#95).

It would be nice to have a new module for administrators to build/pull all images. This would hold a registry of all the docker containers and the names of the corresponding singularity images. The ewatercycle.models.* classes could then get the docker/singularity images from this module, instead of having it hardcoded.

[sverhoeven]

Now using ewatercycle.CFG['singularity_dir'] dir for sif files see https://ewatercycle.readthedocs.io/en/latest/system_setup.html#id2