Check for overflow in multiplot environment

[LucaCiucci]

When in a multicolor environment, throw an exception if the user draws more times than the number of subplots.
Also (I'm not sure this is a good idea) add the option to ignore this check.

Also when in multicolor environment other calls of multiplot shall not be allowed.

In addition to this: can subplots have child subplots? If so, the multiplot guard should have another subplot function or some similar mechanism