Avoid x^0 and Order(x^0) terms together in series expansion.

Avoid x^0 and Order(x^0) terms together in series expansion.

At the branch cut, the series expansions of log(), atan(), and atanh()
assembled pseries objects which contained both x^0 and Order(x^0) terms.
This patch removes the extra x^0 term if the order is 0. It also adds
a GINAC_ASSERT for these kinds of invariants to the pseries ctor and
simplifies the loops in pseries' degree(), ldegree(), eval() and evalf()
member functions.