Solver enhancements

• Fixed issue where ULS shear solver did not manage to find global optimum
• Scripting interface feature drafted
• Fixed bug where sign convention figure did not show in the about tab
• Added a few unit tests with automatic code coverage and continuous integration incorporated
• Fixed inconsistency between bending and disk stresses for non-linear stress-strain
• Stiffness input E_cm greyed-out when not applicable
• Added analysis timers and various speed-ups