lcm on list of numbers; factorise(n)

The JME/Numbas.math function lcm now works on a list of numbers, returning the LCM of all of them.
This also adds a new function Numbas.maths.factorise(n), which returns a list [a_2,a_3,a_5,...], where n = 2^(a_2)*3*(a_3)*5^(a_5)*...
add jme function 'distinct'; fix lcm; document math.factorise