Like the popular E.L. Doctorow novel, Ragtime, the Keegan Theatre’s whimsical and bouncy new production of Steve Martin’s play Picasso at the Lapin Agile is an intricate, quasi-historical journey as seen through the prism of historical figures (Einstein and Picasso in this instance — with a surprise contemporary guest from the future and numerous references to Matisse) at the dawning of a new age of creativity and vision.