#139 metadata dropdown list defined in templates

[rscdaya]

It would be great to be able to define a dropdown list of suggested values for a particular metadata field in a template. Currently you can define a value for a metadata keyword in a template like this:
[[Compounds Keywords>Alkanes]]
but if this could be extended to allow more than one value e.g.
[[Compounds Keywords>Alkanes|alkenes|alkynes]]
that would be great. The syntax for specifying it is up for discussion - the suggestion above is just what sprung to mind...