arguments_dict error messages improved

[JohanMabille]

Mandatory Checklist

Please ensure that the following criteria are met:

• [x] Title of pull request describes the changes/features
• [x] Request at least 2 reviewers
• [x] If new files are being added, the files are no larger than 100kB. Post the file sizes.
• [x] Code coverage did not decrease. If this is a bug fix, a test should cover that bug fix. If a new feature is added, a test should be made to cover that feature.
• [x] New features have appropriate documentation strings (readable by sphinx)
• [x] Contributor has read and agreed with CONTRIBUTING.md and has added themselves to CONTRIBUTORS.md

As a general rule of thumb, try to follow PEP8 guidelines.

Description

This PR improves the error message you get when you try to get a scalar or a pyarray of the wrong type from the argument dictionary object.

Old behavior:

// The following is wrong, it should ask for a pyarray of double
xt::pyarray<int>& mesh_trial_ref = args.m_iarray["mesh_trial_ref"];
// Throws an error whose message is:
// mesh_trial_ref not found

New behavior:

// The following is wrong, it should ask for a pyarray of double
xt::pyarray<int>& mesh_trial_ref = args.array<int>("mesh_trial_ref");
// Throws an error whose message is:
// mesh_trial_ref not found in the dict of pyarray<int> but found in the dict of pyarray<double>

[codecov[bot]]

Codecov Report

Merging #1230 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1230   +/-   ##
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  Coverage   52.29%   52.29%           
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  Files         531      531           
  Lines      109524   109524           
=======================================
  Hits        57275    57275           
  Misses      52249    52249           

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[JohanMabille]

@cekees @zhang-alvin @tridelat this is ready for review.