docs: document Table.alignment analysis
acpt: add scenarios for Table.alignment
tbl: reorder tests
tbl: add Table.alignment getter
tbl: add Table.alignment.setter
tmpl: change core props author to python-docx
docs: document CoreProperties analysis
acpt: add scenario for reading CoreProperties
doc: add Document.core_properties
opc: add OpcPackage.core_properties

Also, organize fixture components separate from fixtures.
rfctr: extract opc.rel module from opc.package
opc: consolidate tests for docx.opc.rel

Somehow tests for Relationships became fragmented between test_package
and test_rels. Move additional tests from test_rels into new test_rel
module.
rfctr: extract opc.part module from opc.package
opc: add OpcPackage._core_properties_part
opc: add CorePropertiesPart.core_properties
rfctr: use relative imports in oxml.__init__
opc: transplant CoreProperties
acpt: add scenario for writing CoreProperties
acpt: add scenario for default CoreProperties part
opc: add CorePropertiesPart.default()