Currently calculations such as 22/7 do not work correctly on the
simulator (which uses double precision floating point). Fix this
by explicitly truncating the strings when needed.
Additionally the current calculate() method has some problems when
the calculation cannot be evaluated since it will needlessly clear out
the calculation. Push calculate (and the exception handling) into the
caller and report errors using the vibration motor instead.
Finally we rename display_output() to the more idiomatic _update().
Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>
21210c5c07