This change merges the four main OpenDC development repositories into a
single Git repository and performs final organization of the repository.

This change prepares the opendc repository for a monorepo setup by
moving all files to the core/ directory. After all repositories have
been merged into this repository, we will move the correct files back.

This change adds a conversion script that allows users to import traces
from the Grid Workload Archive (see http://gwa.ewi.tudelft.nl/) into the
OpenDC database.

This change implements the parts of the Datacenter Scheduling Reference
Architecture as published in SC18.

This commit consists of changes to the database schema such as:

- **Removal of `parallelizability` column in `Task`**
  This field was not used by the simulator and we opted to use a field
  describing the amount of cores the task can run on instead.
- **Removal of `task_id` column in `MachineState`**
  This field did not make sense anymore now that a machine can run
  multiple tasks. Fortunately, this field is not used in the frontend.