This commit adds support for `internals` dictionaries.

These can be defined at any level `variables` can be defined at, and are
used to control internal aspects of the application classes.

Currently support functionality includes:
```
internals:
  custom_executables:
    exec_name:
      template:
      - cmd1
      - cmd2
      use_mpi: true
      redirect: '{log_file}'
  executables:
  - exec_name
  - builtin::env_vars
```

The custom_executables dict allows the YAML to define custom executables
to build experiments with, while the executables list provides a list of
executables that will populate the '{command}' variable.

This commit performs the following tasks:
- applies some additional `*_` syntax for consistency
- abstracts the `get_workspace_*` methods
- hoists an import in the application.py module