Added easy calls to disable/enable collections of functions or
individual functions in the current context. So you can say
to disallow the use of trig functions, or
to disable trig and numeric functions (like log, exp, abs, etc) but
Note that disabling sqrt() does not disable x^(1/2) (this must be done
through disabling the ^ and ** operators), and disabling abs() does
not disable |x| (which must be done by removing the '|' definition
from the Context's parens list).