Add rdo()

[WenjieDu]

What does this PR do?

1. add rdo() to randomly drop observation to create missingness;

Before submitting

• This PR is made to fix a typo or improve the docs (you can dismiss the other checks if this is the case).
• Was this discussed/approved via a GitHub issue? Please add a link to it if that's the case.
• I have commented my code, particularly in hard-to-understand areas.
• I have written necessary tests and already run them locally.

[github-actions]

Pull Request Test Coverage Report for Build 9257637291

Details

• 34 of 36 (94.44%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.7%) to 89.42%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pygrinder/randomly_drop_observations/rdo.py	29	31	93.55%

Totals
Change from base Build 8313032233:	0.7%
Covered Lines:	262
Relevant Lines:	293

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 9257637291

Details

• 34 of 36 (94.44%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.7%) to 89.42%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pygrinder/randomly_drop_observations/rdo.py	29	31	93.55%

Totals
Change from base Build 8313032233:	0.7%
Covered Lines:	262
Relevant Lines:	293

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 9257766003

Details

• 40 of 42 (95.24%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.9%) to 89.632%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pygrinder/randomly_drop_observations/rdo.py	32	34	94.12%

Totals
Change from base Build 8313032233:	0.9%
Covered Lines:	268
Relevant Lines:	299

---

💛 - Coveralls