Fix bug #1931: when creating a new subset, use ReplaceMode

[specmicp]

When a new subset is created, override the mode to use ReplaceMode.

The user does not have to remember to reset everytime the mode manually. The mode is only temporarily overriden for the creation of the new subset.

[codecov]

Codecov Report

Merging #2090 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2090      +/-   ##
==========================================
+ Coverage   87.98%   88.01%   +0.02%     
==========================================
  Files         244      244              
  Lines       22270    22283      +13     
==========================================
+ Hits        19595    19613      +18     
+ Misses       2675     2670       -5     

Impacted Files	Coverage Δ
glue/core/command.py	93.82% <100.00%> (+0.14%)	⬆️
glue/viewers/matplotlib/qt/widget.py	78.40% <0.00%> (-3.41%)	⬇️
glue/viewers/profile/layer_artist.py	95.74% <0.00%> (-0.05%)	⬇️
glue/app/qt/application.py	79.69% <0.00%> (+1.19%)	⬆️

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[astrofrog]

Thanks for the pull request! Unless you have a chance to do this, I will take a look at the test failures early next week to see what we need to fix before merging this.

[specmicp]

The first error: "ERROR at setup of TestGlueApplication.test_new_tabs" is I think a problem with upstream: jupyter/jupyter_client#523

The solution to pass the test (at least on my machine) was to downgrade jupyter_client from 6.0.0 to 5.3.4 (available in pip)

[specmicp]

the failing test glue/viewers/image/qt/tests/test_data_viewer.py is only failing with matplotlib 3.1.3, not matplotlib 3.1.2

Here is the changelog: https://matplotlib.org/3.1.3/users/whats_new.html

[astrofrog]

Looks and works great, thanks! I just rebased to check that tests pass now.