Refactoring alignment api

[RickBrice]

Discussing with @Moult @civilx64 @DesertSpringsCivil about refactoring alignment api into the preferred format on the saikei branch - I see there has been some minor refactoring already. I have some time so I could do a semi-heavy lift and do the desired refactoring.

Just so I'm clear, all of the alignment creation/authoring functions belong under api/alignment in their own py files. All of the interrogation (get) functions belong under util and all go into the one and only alignment.py file. Is this correct?

Test cases follow the same structure in test/api/alignment and test/util?

Any concerns with making breaking changes? I don't have any - all my code that use the alignment api are trivial and can be easily fixed.

The proposed changes are tabulated below.

Also @Moult why did you make _create_geometric_representation public? Is further refactoring required?

Alignment API function	api/alignment/*.py	util/alignment.py
"add_stationing_referent",	x
"add_vertical_layout",	x
"add_zero_length_segment",	x
"clear_layout_segments",	x
"create",	x
"create_as_offset_curve",	x
"create_as_polyline",	x
"create_by_pi_method",	x
"create_from_csv",	x
"create_layout_segment",	x
"create_representation",	x
"create_segment_representations",	x
"_create_geometric_representation", # TODO I know I know	x
"distance_along_from_station",		x
"get_alignment",		x
"get_alignment_layout_nest",		x
"get_alignment_layout",		x
"get_alignment_layouts",		x
"get_alignment_segment_nest",		x
"get_alignment_start_station",		x
"get_axis_subcontext",		x
"get_basis_curve",		x
"get_cant_layout",		x
"get_child_alignments",		x
"get_curve",		x
"get_curve_segment_transition_code",		x
"get_horizontal_layout",		x
"get_layout_curve",		x
"get_layout_segments",		x
"get_parent_alignment",		x
"get_referent_nest",		x
"get_vertical_layout",		x
"has_zero_length_segment",		x
"layout_horizontal_alignment_by_pi_method",	x
"layout_vertical_alignment_by_pi_method",	x
"name_segments",		x
"segment_vertices",		x
"register_referent_name_callback",	x
"update_fallback_position",		x
"get_mapped_segments",		x

[civilx64]

Any concerns with making breaking changes?

None here. My expectation is that it will continue to change until there is a robust UI in blender for full CRUD operations.

I have no exceptions to the way you have things laid out in the table. Maybe one suggestion since you're refactoring anyway is that layout_horizontal_alignment_by_pi_method be shortened to horizontal_layout_by_pi (and similar for vertical).