feat: Register unit without updating config files

[hpohekar]

Register unit without updating yaml file.

    >>> from ansys.units import register_unit
    >>> register_unit(unit="Q", composition="N m", factor=1)
    >>> ureg = UnitRegistry()
    >>> ureg.Q
    _name: Q
    _dimensions: {'MASS': 1.0, 'LENGTH': 2.0, 'TIME': -2.0}
    _composition: N m
    _factor: 1
    _si_units: kg m^2 s^-2
    _si_scaling_factor: 1.0
    _si_offset: 0.0