Parameter() raises TypeError with its documented default allowable_keys=None #110

Found while reviewing #74. Pre-existing on main (b764057); #74 does not change the behavior.

Problem

Parameter.__init__() has allowable_keys: set[str] | None = None, and the docstring lists allowable_keys : set[str] | None, default=None. But the first check rejects anything that is not a set, so the default (or an explicit None) always raises TypeError: `allowable_keys` should be a set of str. The docstring does not say what None should mean. __setitem__() also does key not in self.allowable_keys, which would need a set. The only callers in the package, HH_Param and MKZ_Param, always pass a set.

from PySeismoSoil.class_parameters import Parameter

for kwargs in [{}, {'allowable_keys': None}]:
    try:
        Parameter({'a': 1.0}, **kwargs)
        print('OK')
    except TypeError as e:
        print(f'{type(e).__name__}: {e}')
# TypeError: `allowable_keys` should be a set of str.
# TypeError: `allowable_keys` should be a set of str.
# (expected: the documented default works)

param = Parameter({'a': 1.0}, allowable_keys={'a'})
print(dict(param))
# {'a': 1.0}

Fix

Choose one of these:

1. Make allowable_keys a required keyword argument: change the signature to allowable_keys: set[str] with no default, and change the docstring (parameter and attribute) to allowable_keys : set[str]. This matches what HH_Param and MKZ_Param already do.
2. Or give None a meaning, for example "any key is allowed": skip the key checks in __init__() and __setitem__() when self.allowable_keys is None, and say so in the docstring.

Add a test for the chosen behavior: either that Parameter({'a': 1.0}) raises TypeError because the argument is missing, or that it works and accepts new keys.