1771 feature tiled env var

In this change I make the definition of the tiled client optional. I also use the name db for the client to match earlier versions.

The tiled client is installed iff the following environment variables are defined. These should be defined in the docker run .... command. An For example, this is what I use for my installation of tiled.

--env TILED_URL=http://192.168.169.73:8000 --env TILED_API_KEY=secret

I have checked that the IPython profile starts both when the environment variables are defined, and when they are not