field_provider.py mutable default argument

[SeaTechRC]

Hello,

in the FieldProvider class, there are multiple instances of mutable default arguments.
I have observed, that when using the FieldFriend and creating a field, adding more than one row results in the rows sharing the same points until restarted.
The root cause are mutable default arguments (the list of points given when calling create_row as example), which are then referenced in 2 different instances of Row. When restarting the the program, the rows are restored from a file, in which case, the list of points are instantiated seperately (fixing this shared reference).

The fix is to simply change the function to explicitly create a new list, when no argument is supplied.

[pascalzauberzeug]

@SeaTechRC Thank you for the PR. You are totally correct, mutable defaults can lead to very nasty debugging sessions.
I will merge this now to keep the code safe, but we are currently reworking the whole class anyway.