v0.15.0

Major changes:

• Allow usage of MongoDB instead of AiiDA (and PostgreSQL) (#204).
  With this change, one can run aiida-optimade init --mongo and initialize an AiiDA server and instead of storing the calculated OPTIMADE fields in each Node's extras, they will be stored as a document in a MongoDB database collection, along with other relevant fields for completeness. This makes it possible to serve very large, preferrably static, AiiDA DBs with much greater response speeds and utilizes the MongoDB backend setup and configuration already implemented in the upstream optimade package.

Development updates:

• Use Python 3.8 and clean up Dockerfile (#199, #204).
• Update dependencies, ensuring supporting numpy for the supported Python versions (#203)
• Update invoke tasks to support new docker-compose and server config files introduced through #204 (#205).