LR Scheduler: added iteration granularity, exp0 and epoch0-only options

[lrzpellegrini]

This PR introduces the following options to the LRSchedulerPlugin:

• granularity: how often the scheduler's step() method is called. Valid values are 'epoch' (default) and 'iteration'
• first_epoch_only: if True, the step() method will be called only on the first epoch of each experience
• first_exp_only: if True, the step() method will be called only on the first training experience

In addition, first_epoch_only and first_exp_only can be True at the same time.

This also adds a lot of unit tests to validate the behavior with the new options.

[ContinualAI-bot]

Oh no! It seems there are some PEP8 errors! 😕
Don't worry, you can fix them! 💪
Here's a report about the errors and where you can find them:

tests/training/test_plugins.py:436:13: E122 continuation line missing indentation or outdented
tests/training/test_plugins.py:437:13: E122 continuation line missing indentation or outdented
tests/training/test_plugins.py:437:13: E125 continuation line with same indent as next logical line
tests/training/test_plugins.py:555:13: E122 continuation line missing indentation or outdented
tests/training/test_plugins.py:556:13: E122 continuation line missing indentation or outdented
tests/training/test_plugins.py:556:13: E125 continuation line with same indent as next logical line
4       E122 continuation line missing indentation or outdented
2       E125 continuation line with same indent as next logical line