Define gcd & lcm of single element

[d-torrance]

Previously, gcd and lcm raised an error if we passed a single element. This seems a bit odd, especially when we use parentheses, since it works with any other number of arguments:

i1 : gcd()

o1 = 0

i2 : gcd(3)
stdio:2:3:(3): error: no method found for applying gcd to:
     argument   :  3 (of class ZZ)

i3 : gcd(3, 6)

o3 = 3

i4 : gcd(3, 6, 9)

o4 = 3

After defining gcd(ZZ):

i1 : gcd(3)

o1 = 3

[mahrud]

Number, perhaps?

[d-torrance]

I thought about that. I stuck with ZZ and QQ since those are the ones we have binary methods for. It seems like it might be a bit strange if, say, gcd(3.0) worked but gcd(3.0, 6.0) didn't.