Fix inverted sign in upward derivative filter

[YagoMCastro]

In order to apply the convention of positive upwards, this code:

1. Inverts signal of expected wavenumber at derivative_upward_kernel function;
2. Inverts signal of wavenumber at derivative_upward_kernel function;
3. Invert signal of upward derivative on test_transformations.py

Relevant issues/PRs:
Fixes #454

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[leouieda]

@YagoMCastro thanks for the fix! Could you please add a description to the PR? Under "relevant issues" you should include Fixes #454 so the issue will be linked here and closed automatically when this is merged.

[leouieda]

Just need to fix the placement of the minus sign in the test. Could you also add a comment to the test_derivative_upward_order2 function explaining that we don't need the minus sign there because the second derivative is positive both for z (downward) and upward derivatives?

[leouieda]

Great, well done!

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