Support Tuple and CartesianIndex types when passing pivots

[rittermarc]

Users are likely to have indices as Tuple or CartesianIndex. Functions such as addglobalpivots! only accept vectors. Based on user feedback during the tutorial session, it would be helpful to make the interface more flexible such that the user doesn't have to convert pivot specifications all the time.

[shinaoka]

OK, let us override the function!
I want to avoid using Union to accept these types as this could lead to type instability.