Protect settings and variables

[dionhaefner]

Currently, all settings and variables can be overridden at any time. This might cause subtle bugs, both from inside Veros core routines (e.g. when assigning directly to an array instead of updating its values), setups, or even higher-level code.

It may be worth considering protecting variables from being set to entirely new objects (e.g. by overloading setitem in the Veros class). An equivalent check could be made for settings that cannot be safely changed after setup (this should at least warrant a warning).