DDSXML11 — Allow try_construct in union discriminator and remove const from struct declaration

The union discriminator should also support the try_construct specification. Per XTYPES this may be needed when the union discriminator is an enum.

Moreover, the <struct> element allows a <const> to be defined inside. This is inconsistent with the IDL syntax which disallows that.

add try construct attribute to enum discriminator and disallow const decl inside struct

add try construct, min, max, default and key attribute to union discriminator
add external attribute to collection elements
add bitbound attribute to enum and bitmask types