Implement parallelization options as explicit inputs.

If any parallelization option is already specified in the
`settings['CMDLINE']`, a `DeprecationWarning` is issued. However,
the value from `settings['CMDLINE']` still takes precedence. This
allows us to implement the defaults at the level of the input
port, without having to care about it later on.

The `BasePwCpInputGenerator` is given three class attributes:
- `_ALLOWED_PARALLELIZATION_FLAGS` is a list of tuples
  (flag_name, default, help), of all possible flags
- `_ENABLED_PARALLELIZATION_FLAGS` is a list of flag names that
  are implemented in a particular code.
- `_PARALLELIZATION_FLAG_ALIASES` is used to detect all possible
  variations on a flag name.

TODO:
- Testing & documentation
- Check if flag names are case sensitive. If not, this needs to
  be taken into account when checking for existing flags.
- Set `_ENABLED_PARALLELIZATION_FLAGS` for codes other than pw.x.