MatrixLapack: implement methods required by OOPS

[marcin85PL]

Description

OOPS requires that eckit/maths/Matrix.h exposes:

• void setZero()

• Matrix operator/(const Scalar& s) const

When Eckit is compiled without Eigen, these methods are not available (in eckit/maths/MatrixLapack.h).

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 65.76%. Comparing base (1f6c1f0) to head (f713aff).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #235      +/-   ##
===========================================
- Coverage    65.77%   65.76%   -0.01%     
===========================================
  Files         1131     1131              
  Lines        57406    57406              
  Branches      4337     4337              
===========================================
- Hits         37756    37755       -1     
- Misses       19650    19651       +1     

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[Ozaq]

CI is failing due to a unrelated issue (at least as far as I can tell), I think we can proceed to merge this @wdeconinck Edit: Esp since the CI only fails on one platform with a seemingly unrelated issue.