Add time utils

[ANDREWNGT]

This pull request was motivated by the current cdm.py function being unable to take in day of year (DOY) date formats. It could only work with the traditional yy-mm-dd date formats. Since the CCSDS standards mandate that both formats can be used for CDMs, I have taken the liberty to translate the relevant MATLAB code in the NASA CARA analysis tools. https://github.com/nasa/CARA_Analysis_Tools/tree/master/two-dimension_Pc/Main/TransformationCode/TimeTransformations

I have also retrieved several test CDMs that were released by NASA and intend to add them here for easier reference.

Lastly, I have written a unit test for the time_utils functions that I have added.

Happy to take feedback as I am new to open source development workflows!

[Sceki]

Hi @ANDREWNGT , thank you for the PR, it is much appreciated!

Before I have a deeper look at the code (as soon as I find the time), I wanted to ask bring up two aspects:

1. We still have to write a wiki with some coding style guidelines for the repo, and I am sorry for that. Anyway, I wanted to highlight that in Kessler we identify with Camelcase classes, and lower case with underscores all the methods and variables: could you please also do that here (since I see that you mostly use Camelcase)?

2. We discourage pushing data that is not code to avoid unintended extra space being occupied in the user machine. Are you using all those CDM files that are pushed under the "input" folder in your unit tests? If not, I think you should not push them. If you are using them, I recommend using as little as possible to still satisfy the test, and having them directly into the test code (i.e., similar to here).

Thanks again!

Giacomo

[Sceki]

I have made some comments to review your code. Thank you for your contribution.

Also, please see the comment above, in order to be consistent with the library coding style and organization :)

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[Sceki]

Thanks for the updates @ANDREWNGT !

Could you please also change from camel case to underscore (e.g. not thisVariable but this_variable), for all methods and variables ?

[Sceki]

This looks good to me, I am going to merge it, thanks @ANDREWNGT :)