reorg: extract unitdata/autoshape.py
reorg: extract unitdata/coreprops.py
reorg: extract unitdata/table.py
reorg: extract unitdata/text.py
reorg: extract parts/unitdata/part.py
reorg: extract opc/unitdata/rels.py
reorg: remove duplicate def of PartBuilder
test: refactor test_presentation.py to pytest
rels: remove reltype_ordering functionality

Included Observable mixin, rel._num property, and resequence() on
RelsColl. Refactored rels._next_rId() to remove dependency on _num.
rels: rename rel._target to rel.target
rels: rename rel._reltype to rel.reltype
rels: rename rel._rId to rel.rId
test: refactor test_rels.py to latest stds
rels: fill out test coverage for Relationship
rels: rename _additem() to add_rel()
rels: rename _next_rId to next_rId
rels: factor ._rIds out to property
rels: complete remaining test coverage
rels: break RelColl inherit from Collection
prs: move _slides out of __init__() to lazy load