I recommend this excellent browser add-on:
It is a matter of preference whether to use spaces or tabs for indentation. The important thing is to do it consistently.
However, when it comes to alignment, such as with comments, spaces are objectively better. The reason is that alignment using tabs is dependent on the specific tab size setting, and everyone has a different size setting, This is why the alignment is all screwy on GitHub, which uses 8 space tab width.
Back to indentation, as I said, use whatever style you like, but pick one and use it everywhere (though I think it's not so bad if you want to use a different style in the examples vs. library code). But you have a mixture of indentation styles, which is easy to see on GitHub when using that browser add-on I mentioned. Here is an example:
Here you have used both tabs and four space indents. In LiquidCrystal_NKC.h, the indentation is consistent, but it's consistently 2 spaces.
It seems kind of uptight, but it really does make it more difficult for people to contribute to the code when there isn't a clear prevailing code style for them to follow.