Support storing nsrr_token in config or environment variable

[simon-p-2000]

Added support for storing the nsrr_token in an environment variable or the config file of sleepecg. Since the nsrr_token can be set in multiple ways, the hierarchy for which nsrr_token is used is set as such:

1. using the nsrr_token set via the set_nsrr_token function defined in nsrr.py
2. using the nsrr_token set in an environment variable
3. using the nsrr_token set in the config file

Currently, sleepecg provides no functionality to set the nsrr_token as an environment variable or in the config file. Instead, these actions have to be performed by hand.

In addtion, the test_nsrr.py file was added introducing four test cases. Three cases cover the setting of the nsrr_token via methods 1-3 while the fourth test case covers an unset nsrr_token.

[cbrnr]

Also, I'd prefer the environment variable to be NSRR_TOKEN (all caps), since this is more customary.

[cbrnr]

Once you've addressed these comments, please update the corresponding section in the docs and add an entry to CHANGELOG.md.

[cbrnr]

The config system also needs to be updated in order to recognize nsrr_token as a valid key.

[cbrnr]

A few final comments and this is ready!

[cbrnr]

Thanks a lot @simon-p-2000 🎉!