add a check for readonly mode

[koubaa]

When Mechanical doesn't have an active license, it will be readonly in the UI. When using Pymechanical, there should be an easy way to check. In readonly mode, many mutating APIs don't work, which could be difficult to debug.

pytest based tests should assert that we are not in readonly mode in the fixture for embedding. A single test should be added that runs pymechanical as a subprocess in readonly mode to make sure the API works.

The function to check seems to be MechanicalAPI.Instance.ReadOnlyMode, it should be exposed as a getter on the app class
Message window might also have it

[koubaa]

#440 for listing messages from the "message window". The messages there is not quite as useful as a direct APi for readonly mode