Add `WarmStartingProblem`

[sile]

This PR also has a change that allows that problems have a step 0, i.e., no-cost evaluation, which is used for evaluations during a warm-starting phase.

Command to create WarmStartingProblem

$ kurobako problem warm-starting --help
kurobako-problem-warm-starting 0.2.8
Recipe of `WarmStartingProblem`

USAGE:
    kurobako problem warm-starting <source> <target>

FLAGS:
    -h, --help       Prints help information
    -V, --version    Prints version information

ARGS:
    <source>    Source problem recipe JSON
    <target>    Target problem recipe JSON

$ kurobako problem warm-starting $(kurobako problem sigopt --dim 3 ackley) $(kurobako problem sigopt --dim 3 ackley)
{"warm_starting":{"source":{"sigopt":{"dim":3,"name":"ACKLEY"}},"target":{"sigopt":{"dim":3,"name":"ACKLEY"}}}}

[coveralls]

Pull Request Test Coverage Report for Build 605070309

Warning: This coverage report may be inaccurate.

We've detected an issue with your CI configuration that might affect the accuracy of this pull request's coverage report. To ensure accuracy in future PRs, please see these guidelines. A quick fix for this PR: rebase it; your next report should be accurate.

• 0 of 65 (0.0%) changed or added relevant lines in 3 files are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.07%) to 10.752%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/problem.rs	0	2	0.0%
kurobako_core/src/problem.rs	0	4	0.0%
kurobako_problems/src/warm_starting.rs	0	59	0.0%
<!--	Total:	0	65	0.0%	-->

Files with Coverage Reduction	New Missed Lines	%
kurobako_core/src/problem.rs	1	0.0%
../../../../../usr/share/rust/.cargo/registry/src/github.com-1ecc6299db9ec823/either-1.6.1/src/lib.rs	3	0.0%
<!--	Total:	4		-->

Totals
Change from base Build 604474620:	-0.07%
Covered Lines:	1104
Relevant Lines:	10268

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💛 - Coveralls