add ability to protect sheets and lock/unlock cells

[samueljmurray]

Thanks for the great library! We had a need for this extra feature on a project, so we implemented it and it seems to work well.

It allows you to protect sheets and set cells to be locked or unlocked.

[xou]

Hi,
Thanks for the PR!
Sorry if this is a stupid question, I am not that familiar with the protected sheets feature - Is there a difference between a sheet that is not protected, and a protected sheet where all cells are locked = false?

(I am mainly wondering from an API perspective, whether it would make sense to set the protected bit automatically if there is at least one cell with locked = true).

Thanks!