Rapsd fix

[pulkkins]

This pull request introduces a fix for Issue #208 and adds tests for the utils.spectral.rapsd function.

[codecov]

Codecov Report

Merging #211 (e1141ef) into master (88fa819) will increase coverage by 0.24%.
The diff coverage is 90.62%.

@@            Coverage Diff             @@
##           master     #211      +/-   ##
==========================================
+ Coverage   78.97%   79.22%   +0.24%     
==========================================
  Files         135      136       +1     
  Lines        9890     9903      +13     
==========================================
+ Hits         7811     7846      +35     
+ Misses       2079     2057      -22     

Flag	Coverage Δ
unit_tests	79.22% <90.62%> (+0.24%)	⬆️

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Impacted Files	Coverage Δ
pysteps/utils/spectral.py	71.21% <83.33%> (+32.40%)	⬆️
pysteps/tests/test_utils_spectral.py	100.00% <100.00%> (ø)

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[dnerini]

hello @pulkkins , very nice contribution!
I have a super-minor suggestion below, but otherwise looks good to go!

[loforest]

Hi @pulkkins,
Thanks a lot for this!
I did not have time to review it before being merged.
I briefly looked at it and it all looks fine.