Flip OperatingPoint's alpha default to None and remove its FutureWarning

[camUrban]

Problem Statement

OperatingPoint.__init__ defaults alpha to 5.0 degrees when the parameter is omitted at a positive vCg__E, and issues a FutureWarning (via the _UNSET sentinel) announcing that the default will change to None in v6.0.0. At a vCg__E of 0.0, an omitted alpha already resolves to NaN without the warning, since that resolution matches the future default. The v6.0.0 release needs to complete the flip.

Location(s): pterasoftware/operating_point.py, tests/unit/test_operating_point.py

Proposed Solution

1. Change alpha's default from the _UNSET sentinel to None, so an omitted alpha resolves to 0.0 at a positive vCg__E and to NaN at a vCg__E of 0.0, mirroring beta.
2. Remove the FutureWarning and the sentinel branch for alpha, and update alpha's docstring to describe the None default.
3. Update test_initialization_with_defaults in tests/unit/test_operating_point.py to stop expecting the FutureWarning and to assert that the default resolves to 0.0.
4. If the externalFX_W removal (Replace externalFX_W and boundsExternalFX_W with an external_loads interface #230) has also landed, delete the then-unused _Unset class and _UNSET instance; otherwise leave them for that issue.

Additional Context

This is a breaking results change for any caller who omits alpha and relied on the 5.0 degree default, which is why it is scheduled for the v6.0.0 major release and pre-announced with the FutureWarning.