Changed to using the is_nan() method (#30)

Changed the possibly misleading / confusing boolean check
for checking if a variable is NaN from 'self != self'
to 'self.is_nan()' for inv functions for ComplexField for
c32, c64, and Complex<E>. Under the hood, is_nan() is
still using the 'self != self' check, however this should
now only appear in that definition, and should cause less
confusion.