Support monrhly interval in PIT.

[ChiahungTai]

🌟 Feature Description

Support monthly interval in PIT.

Motivation

1. Application scenario
2. Related works (Papers, Github repos etc.):
3. Any other relevant and important information:

Some markets(ex: Taiwan Stock Market) will provide monthly revenue every month. So that make senese to support monthly in PIT.

Alternatives

N/A

Additional Notes

Related codes

1. dump_pit.py
2. pit/collector.py
3. qlib/data/data.py: LocalPITProvider
4. qlib/utils/init.py: get_period_list, get_period_offset, read_period_data
5. test_pit.py

@bxdd @you-n-g, how do you think? Is there any related code I am missing?

[you-n-g]

@ChiahungTai I reviewed this PR again https://github.com/microsoft/qlib/pull/343/files . And I think you have considered all the related code. Looking forward to your PR :)

Thanks :)

[ChiahungTai]

Thanks for your comment. I will do my best.