Allow single value input for DataStructureDefinition

[phackstock]

When creating a new DataStructureDefinition object it would be nice to specify a single dimension just as a string and not a list which only contains one string, i.e. instead of:

dsd = DataStructureDefinition(..., dimensions=["region"])

it would be nice to do this:

dsd = DataStructureDefinition(..., dimensions="region")

The same goes for DataStructureDefinition().validate(df, dimensions=['region']).