Rename threshhold to threshold

[tsmbland]

Description

Renames the incorrect spelling "threshhold" in three parameters. I've also added a warning if a user attempts to use "asset_threshhold" in the settings file. This isn't required for the other two parameters, see here

Apparently none of these parameters are explained in the documentation, so there's nothing that needs changing in the documentation, but this obviously isn't a good thing!

Fixes #301

Type of change

Please add a line in the relevant section of
CHANGELOG.md to
document the change (include PR #) - note reverse order of PR #s.

• New feature (non-breaking change which adds functionality)
• Optimization (non-breaking, back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (whatever its nature)

Key checklist

• All tests pass: $ python -m pytest
• The documentation builds and looks OK: $ python -m sphinx -b html docs docs/build

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

Attention: Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 71.31%. Comparing base (0355d08) to head (c44d409).

Files	Patch %	Lines
src/muse/sectors/subsector.py	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #447      +/-   ##
===========================================
- Coverage    71.35%   71.31%   -0.04%     
===========================================
  Files           44       44              
  Lines         5878     5881       +3     
  Branches      1152     1153       +1     
===========================================
  Hits          4194     4194              
- Misses        1364     1366       +2     
- Partials       320      321       +1     

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[alexdewar]

LGTM! Good idea to warn users if they use the old option name.