Add `$ kurobako dataset surrogate-optuna-study` command

[sile]

This PR adds a command to build a surrogate model from Optuna study. The built model can be used as a surrogate model based problem (i.e., as the input for the $ kurobako problem surrogate command).

Usage

$ kurobako dataset surrogate-optuna-study -h
kurobako-dataset-surrogate-optuna-study 0.2.8
Builds a surrogate model problem from Optuna studies

USAGE:
    kurobako dataset surrogate-optuna-study [OPTIONS] --problem-name <problem-name> --storage <storage>

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

OPTIONS:
        --max-samples <max-samples>
            Max samples used for building each tree in a random forest [default: 1000]

        --objective-index <objective-index>        Objective value index [default: 0]
        --out <out>                                Output directory [default: surrogates/]
        --problem-name <problem-name>              Problem name
        --storage <storage>                        Optuna storage URI
        --target-study-name <target-study-name>    Target study name (regexp) [default: .*]
        --trees <trees>                            Number of trees in a rando forest [default: 1000]

[coveralls]

Pull Request Test Coverage Report for Build 607464837

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 148 (0.0%) changed or added relevant lines in 2 files are covered.
• 186 unchanged lines in 10 files lost coverage.
• Overall coverage decreased (-1.0%) to 9.729%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/dataset.rs	0	3	0.0%
src/dataset/surrogate.rs	0	145	0.0%
<!--	Total:	0	148	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/trackable-0.2.23/src/macros.rs	2	0.0%
../../../../../usr/share/rust/.cargo/registry/src/github.com-1ecc6299db9ec823/byteorder-1.4.2/src/lib.rs	3	0.0%
../../../../../usr/share/rust/.cargo/registry/src/github.com-1ecc6299db9ec823/byteorder-1.4.2/src/io.rs	5	0.0%
../../../../../usr/share/rust/.cargo/registry/src/github.com-1ecc6299db9ec823/randomforest-0.1.6/src/random_forest/regressor.rs	7	0.0%
../../../../../usr/share/rust/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.7.3/src/seq/mod.rs	9	0.0%
../../../../../usr/share/rust/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.7.3/src/lib.rs	10	0.0%
../../../../../usr/share/rust/.cargo/registry/src/github.com-1ecc6299db9ec823/randomforest-0.1.6/src/random_forest/core.rs	32	0.0%
../../../../../usr/share/rust/.cargo/registry/src/github.com-1ecc6299db9ec823/rand-0.7.3/src/distributions/uniform.rs	38	0.0%
../../../../../usr/share/rust/.cargo/registry/src/github.com-1ecc6299db9ec823/randomforest-0.1.6/src/decision_tree.rs	79	0.0%
<!--	Total:	186		-->

Totals
Change from base Build 605080038:	-1.0%
Covered Lines:	1104
Relevant Lines:	11348

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💛 - Coveralls