lerp (start, end, delta) {
        'use strict'
        return start + ((end - start) * delta)
    }
Perform a linear interpolation operation on two numbers.
@param  {Number} start The start value.
@param  {Number} end   The end value.
@param  {Number} delta The position to interpolate to.
@return {Number}       The result of the lerp operation.