New functions and future proofing

This release adds the following new functions:

• send_key_code: Simulate a keypress on the control. only works if you first lock the keyboard access. Available keycodes can be found in const.py
• set_keyboard_access: lock and unlock the keyboard access
• set_machine_parameter: Change the value of a machine parameter
• get_machine_parameter: Read the current value of a machine parameter
• is_itnc, is_tnc and is_pilot: check which type of control to enable workarounds for differences in implementation

Other changes:

• The main class collected a lot of constant definitions. These wer extracted to const.py to clean up the code. They will be removed from the main class in a future release
• Translation from error codes, execution and program state is now handled in translate_messages.py
• updated lsv2_demo.py to show how to use the new functions
• pytest can now run agains TNC640, iTNC530 and CNCPILOT640