o Load the option definitions with the OptionPool

  o Set the user selected project option values according to the passed Context

  o Process conditionals in the loaded project configuration,
    excepting for the project name and actual option definitions

  o Removed a lot of examples from the default config.

    This is getting moved into more structured documentation, and the
    default config is only presented to the user beside the documentation
    which describes what each thing means.

Renamed _workspaces() -> _list_workspaces(), and changed local
cache of workspaces from __workspaces -> _workspaces.

We only really use the double underscore when there is a concern
for freeing up namespace for public subclassing.

This means we also revert back to the older, simpler one step
loading of the Project. This was broken into multiple steps since
we had introduced the project variants.

Consequently improved _yaml.node_sanitize() to omit a crazy lambda
which had no effect at all on the outcome of the function.

All of the _yaml APIs which deal with a node (dictionary loaded
from YAML) are named _yaml.node_foo(), _yaml.node_bar(), _yaml.node_baz()
etc, except for this one glaring exception.

Also added a missing internal API documenting comment for _yaml.node_validate()

Also updated all callers to _yaml.validate_node() to now call _yaml.node_validate().

With this commit, we first look at the artifacts options in the project
configuration, then fall back on the user configuration if necessary.

Relates to #87

This fixes a regression from 6691d0eb7e20c5872d7a5f439f308ae46019777f.

Instead of hiding it away in the Element constructor, I want
this to be easy to find.