make template location configurable

[JuliaSprenger]

Currently a fixed set of templates is shipped with the DigLabTools version. However, to allow for custom templates a configurable template location is necessary.

[JuliaSprenger]

This feature should introduce a new key in the config.yml named template_folder. Templates in this folder should be used with priority over the ones shipped with DigLabTools. i.e. for a give name of a template first we check a template_folder was specified and if the template exists in that location, otherwise the default location is checked. If the the template does not exist in both an error should be raised.