Default branch for conditional expressions

[kilian]

In conditional expressions such as

foo = if (a > b) then 1 else 2 endif

the else-branch is mandatory, of course. However, if we make it optional, it is less clumsy to define conditional analysis such as

analysis = 
  if (...) then record (foo, value) endif
  and 
  ...

The optional branch should default to 0 for numerics, "" for strings, and true (!) for logicals.

[kilian]

Wit r2335, optional else as well as alsif have been implemented for conditional expression.