add $schema OpenAPI references and apply official CWL JSON schema reference

[fmigneault]

• Add the official CWL JSON schema reference (https://github.com/common-workflow-language/cwl-v1.2/pull/256) as $schema parameter returned in under the OpenAPI schema for the CWL component employed by Weaver (fixes #547).
• Add $schema field auto-insertion into the generated OpenAPI schema definition by CorniceSwagger when corresponding colander.SchemaNode definitions contain a _schema = "<URL>" attribute (fixes #157).

[codecov[bot]]

Codecov Report

Merging #551 (7f600a4) into master (f07cfea) will increase coverage by 0.03%. Report is 2 commits behind head on master. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #551      +/-   ##
==========================================
+ Coverage   84.38%   84.42%   +0.03%     
==========================================
  Files          79       79              
  Lines       17600    17633      +33     
  Branches     2670     2674       +4     
==========================================
+ Hits        14852    14886      +34     
  Misses       2019     2019              
+ Partials      729      728       -1     

Files Changed	Coverage Δ
weaver/datatype.py	78.81% <100.00%> (ø)
weaver/processes/utils.py	79.77% <100.00%> (+0.07%)	:arrow_up:
weaver/wps_restapi/colander_extras.py	83.03% <100.00%> (+0.49%)	:arrow_up:
weaver/wps_restapi/swagger_definitions.py	99.82% <100.00%> (+<0.01%)	:arrow_up: