fields: tiny change to docstring
schema: implement lazy creation of dump function

a schema instance's custom dump function is now created at first access,
not at schema instantiation.
Merge branch 'feature/lazy-dump-function-creation' into develop
tests: remove unnecessary import
implement lazy eval of schema inst for linked-obj-fields ...

and restructure/add tests
Schema: lazy eval of dump function: add additional error info
util: fix docstring
docs: update project info
Update changelog.