gmpxx implicit conversion between types

[Marc Glisse]

What I eventually implemented is:
when the addition of an element of type A and an element of type B yields 
an element of type B: keep the conversion A->B implicit, and make the 
reverse conversion B->A explicit.

In practice, this means that the following conversions are now explicit:
mpq_class -> mpz_class
mpf_class -> mpz_class
mpf_class -> mpq_class

There is still time to complain and get me to change that :-)

-- 
Marc Glisse