Remove the default "recursive __call__".

Now if one needs to use an expression as an operator, the `rcall` method should
be used. The use of the default `__call__` method was deemed too confusing for
novice users.
Remove wrong dependencies on the recursive __call__.
A doctest for rcall.
Remove wrong dependencies on "recursive __call__".
Merge pull request #1857 from Krastanov/rcall_notdefault

Remove the default "recursive __call__".