Bug Fix for Orthotropic Rectangular Plates

• Bug fix for orthotropic rectangular plates. The stiffness was slightly off on rectangular plates in version 0.0.55. Prior versions were not affected. Version 0.0.55 was the latest version for less than 24 hours, so this bug shouldn't have affected most models users have built.
• Added unit testing for in-plane (membrane) behavior, and stiffness modification factors to identify issues faster in the future. PyNite solutions matched theoretical solutions within 0.1% for uncracked sections, and within 2% for cracked sections. Very good performance for in-plane behavior.