fix execution of WPS remote providers and corresponding metadata

[fmigneault]

Changes

• add auto-handling of recoverable invalid XML characters (<, <=, etc.)
  Help support remote providers, temp fix until properly resolved by pywps/finch
  (see Escape XML responses geopython/pywps#628)
• add end-to-end test of WPS provider registration, process description, execution and result retrieval
  (same process as tested by https://github.com/Ouranosinc/pavics-sdi/blob/master/docs/source/notebook-components/weaver_example.ipynb, but using mocked 'remote' WPS)
• block provider operations when not applicable (Weaver ADES/DEFAULT config modes)
• detect and block (better message of cause) invalid combinations of Weaver mode vs Remote provider execution
• fix HYBRID mode not calling WPS remote provider CWL handler as expected
• refactor provider functions into separate utils vs views modules for easier reuse and to avoid circular imports
• fix URLs reported in links by processes nested under a provider Service.
• fixes to OpenPAI schema definitions and resolution during validation

Fixes

• Fixes Schema validation error during providers listing when there are no providers #339
• Fixes WPS provider execution failing parsing of body #340
• Fixes Providers post or retrieval issue  #341

[codecov]

Codecov Report

Merging #342 (30e4c5e) into master (d229afd) will increase coverage by 3.70%.
The diff coverage is 87.79%.

@@            Coverage Diff             @@
##           master     #342      +/-   ##
==========================================
+ Coverage   74.09%   77.79%   +3.70%     
==========================================
  Files          62       63       +1     
  Lines        9863     9981     +118     
  Branches     1487     1508      +21     
==========================================
+ Hits         7308     7765     +457     
+ Misses       2133     1732     -401     
- Partials      422      484      +62     

Impacted Files	Coverage Δ
weaver/processes/execution.py	82.28% <ø> (+4.34%)	⬆️
weaver/wps_restapi/processes/__init__.py	100.00% <ø> (ø)
weaver/tweens.py	77.94% <50.00%> (ø)
weaver/processes/utils.py	78.47% <55.55%> (+0.09%)	⬆️
weaver/processes/wps_package.py	71.49% <62.50%> (+4.46%)	⬆️
weaver/datatype.py	70.13% <71.42%> (+1.96%)	⬆️
weaver/xml_util.py	92.30% <92.30%> (+3.41%)	⬆️
weaver/wps_restapi/providers/providers.py	79.67% <92.45%> (+25.03%)	⬆️
weaver/exceptions.py	86.00% <100.00%> (+0.14%)	⬆️
weaver/processes/convert.py	77.11% <100.00%> (+1.05%)	⬆️
... and 19 more

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[dbyrns]

I have a few questions as usual, but overall everything looks fine :)