Stub to allow loading models without workflow

[tovbinm]

Related issues
We are starting a sequence of PRs on allowing model loading without workflows...

Describe the proposed solution
Stub to allow loading models without workflow

[wsuchy]

LGTM

[codecov]

Codecov Report

Merging #269 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #269      +/-   ##
=========================================
- Coverage   86.41%   86.4%   -0.01%     
=========================================
  Files         318     318              
  Lines       10458   10461       +3     
  Branches      349     555     +206     
=========================================
+ Hits         9037    9039       +2     
- Misses       1421    1422       +1

Impacted Files	Coverage Δ
.../op/stages/impl/feature/SubstringTransformer.scala	100% <ø> (ø)	⬆️
...main/scala/com/salesforce/op/OpWorkflowModel.scala	93.97% <100%> (+0.07%)	⬆️
.../src/main/scala/com/salesforce/op/OpWorkflow.scala	87.5% <100%> (ø)	⬆️
...cala/com/salesforce/op/OpWorkflowModelReader.scala	90.56% <100%> (+2.33%)	⬆️
...es/src/main/scala/com/salesforce/op/OpParams.scala	85.71% <0%> (-4.09%)	⬇️

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