PGeometric = function(start, step, repeats) {
    Pattern.call(this);
    this.start = start;
    this.value = start;
    this.step = step;
    this.repeats = repeats;
    this.position = 0;
}
Geometric sequence.  Multiplies a running total by a value on each next
call.

@constructor
@extends Pattern
@param {Number} start Starting value.
@param {Pattern|Number} step Value to multiply by.
@param {Number} repeats Number of values to generate.