[MRG] add --output-dir as alias for every --outdir

[ctb]

This PR adds --output-dir as an alias for --outdir in commands that take --outdir to directing output to a directory.

Fixes #1812

• consider adding --outdir to every --output-dir command too
• maybe create an issue to swizzle the variable name to output_dir instead of outdir.

[codecov]

Codecov Report

Merging #1817 (0d9c88f) into latest (27826d8) will increase coverage by 8.06%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           latest    #1817      +/-   ##
==========================================
+ Coverage   82.25%   90.31%   +8.06%     
==========================================
  Files         119       89      -30     
  Lines       12918     8727    -4191     
  Branches     1727     1727              
==========================================
- Hits        10626     7882    -2744     
+ Misses       2026      579    -1447     
  Partials      266      266              

Flag	Coverage Δ
python	90.31% <91.66%> (ø)
rust	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/sourmash/cli/compute.py	100.00% <ø> (ø)
src/sourmash/cli/sig/split.py	100.00% <ø> (ø)
src/sourmash/cli/sketch/dna.py	100.00% <ø> (ø)
src/sourmash/cli/sketch/protein.py	100.00% <ø> (ø)
src/sourmash/cli/sketch/translate.py	100.00% <ø> (ø)
src/sourmash/sig/__main__.py	91.75% <83.33%> (ø)
src/sourmash/command_compute.py	93.35% <100.00%> (ø)
src/sourmash/command_sketch.py	88.00% <100.00%> (ø)
src/core/src/ffi/index/mod.rs
src/core/src/index/sbt/mhbt.rs
... and 28 more

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[ctb]

Ready for review @sourmash-bio/devs

[bluegenes]

Lgtm!

[ctb]

thanks!