Use EXPERIMENT.EXPNAME for the namelist name if no template is provided.

For a custom experiment, one might not want to use a template, but provide your own namelist and modify the common/expdef.yml file directly. When doing so, it is convenient to not use the DEFAULT.TEMPLATE parameter for determining the namelist name, but instead use EXPERIMENT.EXPNAME.

Alternatively, one can always use the latter and by default set the experiment name to the template name.

This can be addressed together with #14 (closed).