Feature: Auto-populate selectivity function parameter columns in gear_params

[gustavdelius]

This PR implements auto-population of selectivity function parameter columns in the gear_params S3 class.

Summary of Changes

1. Auto-population in check_gear_params:
   Updated check_gear_params to automatically check the arguments of functions stored in sel_func and add any missing arguments as columns in the gear_params object with NA values.
2. Copy and Default-filling Flow:
   Since the columns are now automatically created, validGearParams's copy-over and default-filling flow works out-of-the-box when assigning the gear_params object back to the MizerParams object.
3. Unit Tests:
   Added a new unit test block in tests/testthat/test-setFishing.R to verify that setting sel_func <- "sigmoid_length" dynamically creates l25 and l50 columns.
   Adapted tests/testthat/test-calc-selectivity.R to strip the subclass prior to parameter deletion testing.