Adding custom spectrum function for random waves

[adimako]

Mandatory Checklist

Please ensure that the following criteria are met:

• Title of pull request describes the changes/features
• Request at least 2 reviewers
• If new files are being added, the files are no larger than 100kB. Post the file sizes.
• 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.
• New features have appropriate documentation strings (readable by sphinx)
• 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

[adimako]

@cekees @ejtovar this is a development I had made for a project that required the use of a custom spectra for random waves calculations. I was planning to submit at some point, but as I am now leaving, I cannot delay it any more. I will probably write some tests soon not to reduce coverage

[cekees]

Looks, good. Thanks @adimako. I will do a release soon after this goes in.