Create select menu from currently selected variables
This is a special hack that supports variable names as options
The string _var_ is prepended to the name to flag this as
a special property.
Extension of VelocityContext class that is used by LAS to store
parameters that will be used by the LAS Velocity templates.
There are two contexts in LAS -- a session TemplateContext, which
is available to every template in a given session, and a local
TemplateContext which is only available to the current template.