batman explosion, Interval changes

added batman explosion class, added to demo
made interval constructor parameter named optional parameters.
also added a minimum parameter to the normal distribution to account for
values that should be nonnegative (set min to 0) or positive (set to 1)
particle emitter, updated demo, ...

particle emitter - creates particles using intervals. allows for
creating a particle 'type' that also has variation

added more explosion types to the demo: some cool parametric curves

made the batman curve equations a bit faster

added errors if explosionmixin/particle emitter constructors are called
without necessary intervals

made use of interval for the color in trail mixin class