Features/storage validation

[jakob-wo]

This PR adds a validation of the stratified thermal storage model to the documentation.
The code behind it (creates the figure) is added to the example section.

[lgtm-com]

This pull request introduces 1 alert when merging abda094 into a70c542 - view on LGTM.com

new alerts:

• 1 for Module is imported with 'import' and 'import from'

[jakob-wo]

@c-moeller please have a look at the documentation and let me know if you understand how the validation was conducted from what I wrote so far.

[c-moeller]

@jakob-wo Thank you for the validation! It reads good and understandable. I've just made some small remarks.

Could you give a final statement about the storage model below the figure? Does it calculate losses in an appropriate way for energy system analysis? And why are the measurement data fluctuating that much? Measuring inaccuracy?

And one final question: You did the validation for a quite high storage level. Maybe you can add a line, why this is sufficient. Or should we contrast a validation with lower storage levels?

[jakob-wo]

@c-moeller, thank you for the review!

I added the missing text in the results section and considered your suggestions in the changes.
Please, have a look at the new text and let me know if it answers the questions sufficiently.

[c-moeller]

Looks good. Thank you.
I've just made some small change in the table. Sorry, maybe it was not clear what I meant.

[jakob-wo]

@c-moeller: Great! I am going to merge it then. Thank you.