MRG: panic when FSStorage::load_sig encounters more than one Signature in a JSON record

[ctb]

This PR was originally about debugging sourmash-bio/sourmash_plugin_branchwater#445, but that's going to require more work to fix properly. For now, I would like to nominate it for merge because sourmash fails silently in this situation, and that's Bad.

In brief, the main thing this PR does is panic with an unimplemented! when FSStorage::load_sig encounters more than one Signature in a JSON record.

This PR also adds a bit of documentation to InnerStorage, per the bottom of this comment.

---

The problem at hand: when loading a SigStore/Signature from a Storage, sourmash only loads the first one and ignores any others.

sourmash/src/core/src/storage/mod.rs

Lines 34 to 38 in 26b50f3

	fn load_sig(&self, path: &str) -> Result<SigStore> {
	let raw = self.load(path)?;
	let sig = Signature::from_reader(&mut &raw[..])?
	// TODO: select the right sig?
	.swap_remove(0);

This results from the concept of a Signature as containing one or more sketches; the history of this is described here, and it leads to some interesting silliness in the Python layer.

The contrapositive is that, in Rust, a single Signature can include multiple sketches, e.g. with different ksizes. So this works fine for the wort case where we have a single .sig file with k=21, k=31, k51.

Note that the Python layer (and hence the entire sourmash CLI) fully supports multiple Signatures in JSON: this is well tested and well covered behavior. The branchwater plugin runs into it because it is using the Rust layer and the API is not fully fleshed out there.

---

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 86.47%. Comparing base (6ae9cd3) to head (abc01c1).

Files with missing lines	Patch %	Lines
src/core/src/storage/mod.rs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           latest    #3333      +/-   ##
==========================================
- Coverage   86.47%   86.47%   -0.01%     
==========================================
  Files         137      137              
  Lines       16086    16090       +4     
  Branches     2219     2219              
==========================================
+ Hits        13911    13914       +3     
- Misses       1868     1869       +1     
  Partials      307      307              

Flag	Coverage Δ
hypothesis-py	25.43% <ø> (ø)
python	92.40% <ø> (ø)
rust	62.27% <85.71%> (+0.01%)	⬆️

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[ctb]

@luizirber @bluegenes thoughts welcome!