Use an Enum for space types

[jrmaddison]

class SpaceType(Enum):
    PRIMAL = 1.0 + 1.0j
    CONJUGATE = 1.0 - 1.0j
    DUAL = -1.0 - 1.0j
    ANTIDUAL = -1.0 + 1.0j

(or a normalized version, i.e. the fourth roots of -1) has a nice structure, so that space types can be mapped

SpaceType(space_type.value.conjugate())  # -> conjugate
SpaceType(-space_type.value)  # -> dual
SpaceType(-space_type.value.conjugate())  # -> antidual

Also take the opportunity to rename 'conjugate dual' to 'antidual'. 'Antidual' has slightly less symmetry, but 'conjugate' still seems preferable to 'antiprimal'.

[jrmaddison]

Update will need to be backwards compatible, meaning that string space type arguments (including "conjugate_dual") will need to be handled correctly.