[BLD] EBNF and XML for External atoms

Harold,

I just noticed that the copy of the condition language EBNF in section 
2.6.2 was not corrected for "External(ATOMIC)" (to "External(Atom|Frame)").

The PS2XML table has not been changed either, but since it refers to an 
"atomicexpr" that is not further specificied, one has to rely on the 
EBNF anyway; so, it does not really need be changed there.

Cheers,

Christian