Modified ProxyList semantics for the special case of Enums.

This change allows for exploiting that flufl.enum.Enum.__getitem__
resolves strings naming enumerations to allow for a more consise
string-based syntax, while still preserving the richness of Enums.
For instance, SRSDG645.channel can be invoked as ddg.channel['A']
instead of ddg.channel[ddg.Channels.A], by using that
ddg.Channels['A'] == ddg.Channels.A.
Fixed SRS DG645 bugs, added ouput functionality.