Option to replace maps by constant dummy map

[HenningSE]

What does the code in this PR do / what does it improve?

This PR adds a URLConfig protocol, creating a dummy map returning constant values. The DummyMap code is taken from WFSim.

[ramirezdiego]

Thanks, @HenningSE! Merging now.

[coveralls]

Pull Request Test Coverage Report for Build 10898531897

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 6 of 17 (35.29%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 80.188%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
fuse/context.py	6	17	35.29%

Totals
Change from base Build 10830815049:	-0.2%
Covered Lines:	2562
Relevant Lines:	3195

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💛 - Coveralls