Add optional reference point to problem

[hvy]

Introduces a reference point field reference_point to Kurobako's problems. Prior to this change, the reference points could be specified via attrs which was somewhat ad-hoc. This PR also adds an actual reference point to ZDT for demonstrational purpose.

This field is

• read from when plotting hypervolumes.
• is optional. If None, plotting will fall back to a hard-coded point.

[coveralls]

Pull Request Test Coverage Report for Build 475724182

• 0 of 21 (0.0%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.03%) to 14.254%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
kurobako_problems/src/zdt.rs	0	1	0.0%
src/record/study.rs	0	2	0.0%
kurobako_core/src/problem.rs	0	18	0.0%
<!--	Total:	0	21	0.0%	-->

Files with Coverage Reduction	New Missed Lines	%
src/record/study.rs	1	0%
<!--	Total:	1		-->

Totals
Change from base Build 474834604:	-0.03%
Covered Lines:	1098
Relevant Lines:	7703

---

💛 - Coveralls

[hvy]

PTAL.

[hvy]

Thanks for your suggestions, applied and PTAL.

[hvy]

Codecov upload failure by CI.

with:
    github-token: ***
    parallel: true
    path-to-lcov: /tmp/grcov-report-vrio2uc3gci
    coveralls-endpoint: https://coveralls.io
Using lcov file: /tmp/grcov-report-vrio2uc3gci
Error:  "2021-01-11T01:04:41.932Z"  'error from getOptions'
Error: Error: Command failed: git cat-file -p f7c81312770f389e4b17f8b8f63f879a9e55324b
fatal: Not a valid object name f7c81312770f389e4b17f8b8f63f879a9e55324b

Looks similar to the "race condition" issue with checkout@v1 but I'm not entirely sure since master haven't been updated. How about trying checkout@v2?

[sile]

How about trying checkout@v2?

Sounds good. Thank you for your suggestion.