Workspace default templates and templates as name-strings or paths in experiment config

[marshrossney]

At the moment an experiment config has to have a template field which maps to the Template dataclass containing attributes template, checkout and directory which mirror argument names in cookiecutter.main.cookiecutter.

However, checkout and directory are not required and are not even used if the template is a filepath. This leads to configuration files that look like

name:
    template:
        template: /path/to/template

which is annoying and confusing. If the template is one that has been added to the named templates file then that should also be accepted in the experiment config as template: name.

Also, .pyrex_workspace.yaml should contain a workspace-wide default template to save space in the experiments config file.