MRG: enforce a single scaled on a `CollectionSet`

[ctb]

Addresses https://github.com/sourmash-bio/sourmash/issues/3386

https://github.com/sourmash-bio/sourmash/pull/3387 changed select to update Record.scaled to the desired scaled value. This PR changes CollectionSet to require that all scaled values be the same, which can now be achieved by running select ;).

It also adds a new method Collection::min_max_scaled() which makes it easy to retrieve scaled for a Collection.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 86.46%. Comparing base (97e7808) to head (2ce356d). Report is 1 commits behind head on latest.

Files with missing lines	Patch %	Lines
src/core/src/collection.rs	50.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## latest #3397 +/- ## ========================================== - Coverage 86.47% 86.46% -0.01% ========================================== Files 137 137 Lines 16095 16100 +5 Branches 2219 2219 ========================================== + Hits 13918 13921 +3 - Misses 1870 1872 +2 Partials 307 307 ``` | [Flag](https://app.codecov.io/gh/sourmash-bio/sourmash/pull/3397/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sourmash-bio) | Coverage Δ | | |---|---|---| | [hypothesis-py](https://app.codecov.io/gh/sourmash-bio/sourmash/pull/3397/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sourmash-bio) | `25.43% <ø> (ø)` | | | [python](https://app.codecov.io/gh/sourmash-bio/sourmash/pull/3397/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sourmash-bio) | `92.40% <ø> (ø)` | | | [rust](https://app.codecov.io/gh/sourmash-bio/sourmash/pull/3397/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sourmash-bio) | `62.29% <50.00%> (-0.01%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sourmash-bio#carryforward-flags-in-the-pull-request-comment) to find out more.

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[ctb]

Ready for review @luizirber @bluegenes !