Adding StreamScaler unit model, along with the test and rst file.

[tannerpolley]

Fixes

Adding a python file that contains a new unit model "StreamScaler" along with a test file and a rst file.

Summary/Motivation:

Adding a useful unit model build by Douglas Allen. This pull request was a task for me to learn and practice the skills necessary to become proficient and code/software management

Changes proposed in this PR:

• Changing the default timeout timer in the build.py file when building the docs. It always times out for me at the default 180 seconds

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1. I agree my contributions are submitted under the license terms described in the LICENSE.txt file at the top level of this directory.
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[andrewlee94]

A few comments fro ma first pass - the main thing is that the docs need to tell the user more about what this model does.

However, I think we should consider a different name for this model as including Scaler in the name will probably cause confusion with the new Scaler classes for numerical scaling.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 80.88235% with 13 lines in your changes missing coverage. Please review.

Project coverage is 77.06%. Comparing base (b032b22) to head (89e326b).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
idaes/models/unit_models/stream_scaler.py	80.59%	12 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1517      +/-   ##
==========================================
- Coverage   77.06%   77.06%   -0.01%     
==========================================
  Files         386      387       +1     
  Lines       62418    62486      +68     
  Branches    10230    10236       +6     
==========================================
+ Hits        48101    48153      +52     
- Misses      11884    11900      +16     
  Partials     2433     2433              

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[dallan-keylogic]

Looks good now.

[bpaul4]

Besides my minor comments below, I think a test with a multiplier value greater than 1 would be useful to confirm the unit behavior. For example, if feed has a flow of 1 and the multiplier value is 2, does the outlet stream have a flow of 0.5 or is just the scaling factor 0.5? Also a check on a multiplier value of 0, whether values less than 1 should be allowed, or if only integer values should be allowed.