Improved GUI and error handling

• NEW "Clear Plots" action has been added to the context menu
• IMPROVED single quotes are now removed from name segments in the tree view
• IMPROVED a RuntimeError is raised when an errors occurs in CVode
• IMPROVED exceptions are raised for undefined declaredType, illegal combinations of causality and variability, and missing shared libraries
• IMPROVED communicationPoint is now calculated as n_steps * step_size to avoid accumulation of numerical errors