Schema-salad

Add parameter to Loader to control cache-to-file behavior (316) tetron
Schemas that are not derived from metaschema can use 'Any' (317) tetron
Don't fail linkchecking validation when null (318) tetron
allow newer rdflib-jsonld 321 mr-c

Allow ruaml.yaml up to version 0.16.5 (from 0.16.0) 314 phaverty

More java codegen fixes. 305 306 307 308 309 jmchilton
Correct attribute for "relation" on link elements 310 milosdanilov
Goodbye Python2 311 312 mr-c

Major thanks to jmchilton for his contributions of a working Java codegen for schema-salad!

https://github.com/common-workflow-language/schema_salad/pull/299 & https://github.com/common-workflow-language/schema_salad/pull/304 and https://github.com/common-workflow-language/schema_salad/pull/300

`schema-salad-doc` also got more options to customize docs in https://github.com/common-workflow-language/schema_salad/pull/298 via jmchilton

tom-tan provided many contributions to improve the error reporting
https://github.com/common-workflow-language/schema_salad/pull/284 Tree structured exception
https://github.com/common-workflow-language/schema_salad/pull/286 Use structured exception for codegen
https://github.com/common-workflow-language/schema_salad/pull/288 Use ValidationException rather than RuntimeError and ValueError
https://github.com/common-workflow-language/schema_salad/pull/287 Refactoring of accessors for SourceLine
https://github.com/common-workflow-language/schema_salad/pull/290 Use ValidationException instead of RuntimeError and ValueError
https://github.com/common-workflow-language/schema_salad/pull/293 Refactor SchemaSaladException and to_one_line_messages

Developer documentation for schema-salad itself: See https://schema-salad.readthedocs.io/en/latest/
https://github.com/common-workflow-language/schema_salad/pull/291 Build the docs mr-c
https://github.com/common-workflow-language/schema_salad/pull/292 use the RTD theme for the docs mr-c

Misc fixes
https://github.com/common-workflow-language/schema_salad/pull/297 allow setting relative values from mr-c
https://github.com/common-workflow-language/schema_salad/pull/294 update black formatting mr-c
https://github.com/common-workflow-language/schema_salad/pull/289 Python 3.8 testing mr-c
https://github.com/common-workflow-language/schema_salad/pull/302 Appveyor has real Python 3.8 support now mr-c

Fix typo breaking internal links 285 by tetron (and a mea culpa from mr-c !)

Fix doc generation refactor t -> entry 282 by tetron
Refactor SourceLinemakeLead 283 by tom-tan
root the exception hierarchy in SchemaSaladException 280 by mr-c with help from manabuishii
base_url parameter to urljoin shouldn't be optional. 279 by tetron
tests: do not lower case the path 277 by mr-c
omit metaschema.py from the coverage check 275 by mr-c