add can_evalf method

The is_number property does not strictly mean that something will
evaluate to a number with evalf since it might contain undefined
functions with numerical arguments. If can_evalf is True it means
that the object can be computed *with precision* to a number (unless
the strict flag is False).
simplify Integral.free_symbols calc; clarify test
7619: fix recursion error on nested Min

Alternatively, instead of testing if x is a number, if the
expression involving the Min were denested as 2*Min(a,b)->Min(2*a,2*b)
the recursion error would not have occured, either...but one might not
want the unnested expression.
clarify is_number related issues; notes in test_basic