Add SheetCollection class

[rap2hpoutre]

Create a class SheetCollection that extends Collection with nothing more for now (so an empty class). It prepares the evolution of export/import system. See #24 (comment)

[shoutoutloud]

Hi. I can be helpful. What should I do?

[rap2hpoutre]

Thank you for your proposition! This ticket is a tiny task, but it prepares a greater feature!

Some more context: currently, we can only import and export one sheet. It's OK for simple cases, but it could be better if optionally, we could import/export more sheets. The original ticket is here: #29. After posting this ticket, I had a long (and really interesting) discussion with @Harti in this PR: #24, and we created a roadmap, thanks to our discussion, in this comment: #24 (comment).

So, I think you have to :

• Read briefly the original tickets (mentioned above) to understand the context.
• Create an empty SheetCollection class that extends Collection in src forder.
• Send a PR when you created the file.
• After that, if you want, we could tackle together the next issue: Export a collection or a sheet collection #35 (which is a bit more complex).

Feel free to ask me anything!

[shoutoutloud]

@rap2hpoutre Thank you for elaborating things.I can create the SheetCollection class but I may not be of much help after that as I am just a newbie.

[rap2hpoutre]

No problem, it's OK!

[rap2hpoutre]

See: #36

[rap2hpoutre]

Fixed, available in v0.5.0 thx!