additional 0.12.x core thoughts

[bluegenes]

from efforts in sourmash-bio/sourmash_plugin_branchwater#197 and #2948

• consider allowing missing paths when loading collections and reporting the number missed or failed to the user. Force to allow failures?
• Can we load a manifest as a collection directly?
• Would it be worth adding a sketchtype option to Selection to explicitly select on MinHash vs LargeMinHash vs HLL sketches?
• How should we handle loading multisig files? See EXP: Show multisig loading issue sourmash_plugin_branchwater#202
• should we add a direct loading function for LargeMinHash?
• should we add Manifest scaled selection for LargeMinHash?
• or, can we have a direct loader that loads MinHash or LargeMinHash, since they have nearly identical usages?
  (... I wasn't really sure how to work with largeminhash)