Update lombscargle.py

[hamzaallen]

Define two functions that calculate the rms and rms error of the spectrum over a specific range of frequencies.

Relevant Issue(s)/PR(s)

Provide an overview of the implemented solution or the fix and elaborate on the modifications.

Is there a new dependency introduced by your contribution? If so, please specify.

Any other comments?

[pep8speaks]

Hello @hamzaallen! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file stingray/lombscargle.py:

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[matteobachetti]

Thanks @hamzaallen! A very useful addition to Stingray!

To verify that the code works, it would be good to have one or more tests added to test_lombscargle.py.
In test_powerspectrum.py we have a few tests for the equivalent function in Powerspectrum and AveragedPowerspectrum, you can use those as templates.

Also, compute_rms does not have a docstring and we require all user-facing methods to be properly documented

[matteobachetti]

Please add a docstring to the function