TYP: Annotate volumeutils

[effigies]

This PR has two significant issues that required changes:

1. Recoders set instance variables based on their input parameters. These variables are always dict-like, but we can't know their names. Adding and annotating __getattr__ resolves this.
2. The DtypeMapper used to allow for Recoders with numpy dtypes was dict-ish, but not quite enough to make annotating it straightforward. Subclassing from dict was sufficient.

Otherwise, it's a fiddly file, so it needs a careful check that I haven't changed the logic or missed renaming a variable.

[codecov]

Codecov Report

Base: 92.16% // Head: 92.16% // No change to project coverage 👍

Coverage data is based on head (b3c3472) compared to base (b3c3472).
Patch has no changes to coverable lines.

❗ Current head b3c3472 differs from pull request most recent head 92c90ae. Consider uploading reports for the commit 92c90ae to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1189   +/-   ##
=======================================
  Coverage   92.16%   92.16%           
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  Files          97       97           
  Lines       12337    12337           
  Branches     2534     2534           
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  Hits        11371    11371           
  Misses        645      645           
  Partials      321      321           

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[ZviBaratz]

Sorry, busy couple of days ahead. Will review beginning of next week if it's nothing urgent.

[effigies]

Please do not consider any review requests urgent. I'm treating it as a "this is ready" notification.

[effigies]

Thanks, Zvi!