Create a stub function for bfast0n

[GreatEmerald]

Initial implementation of the bfast0n function.
Fixes #20

[GreatEmerald]

Fixes #20

[GreatEmerald]

The basics should work now, testing welcome.

There is one issue in that it displays the index of the change in the bfastpp object, but we don't keep it around, so we don't actually know when the change happened. This can be fixed either via bfast2/strucchangeRcpp#15 or by returning both the bfastpp and the breakpoints object. I prefer the former because the bfastpp object by itself is not really that interesting for users. A yet another approach that doesn't depend on strucchange is to do extra parsing in the function of the bfastpp object and then modify the breakpoints result to show the correct corresponding date.

[github-actions]

Due to lack of activity on this pull request, it may now be merged without review.

[GreatEmerald]

Tested and it seems to work. For the issue, let's discuss in the corresponding issue.