Update format of anaconda-project yaml files

[jlstevens]

The existing format used in the anaconda-project yamls was designed to work as an environment yaml as well as a project specification. Unfortunately, this format no longer works properly with anaconda-project lock for newer versions of anaconda-project: https://github.com/Anaconda-Platform/anaconda-project/issues/320

All new anaconda-project yamls should use the backwards compatible format suggested in that issue and old project yamls would ideally be updated too to avoid confusion when updating projects and their lock files.

[jlstevens]

The suggested format has been tested for the boids project here which means boids can be used as our reference when updating the project files everywhere else.

[ablythed]

Done in https://github.com/pyviz-topics/examples/pull/167.