Feedback on API, project=, projection=, other?

[zerothi]

Describe the issue
We have many routines which allows various return values.

For instance we have:

    def inner(
        self,
        ket=None,
        matrix=None,
        projection: Literal["diag", "atoms", "basis", "matrix"] = "diag",
    ):

Now this projection argument is a bit annoying to me, we changed from sum to this, but I don't think it is sufficient.
So what would our users expect, and how should they look like?

There are also interpretation differences between many parts of sisl where the keywords are not consistent.

So, should we rename this? Should different projections have specific methods?
The reason for changing from sum was that it became annoying to have 2 flags, that were mutually exclusive.

Also, when we get there, the different allowed values should be streamlined.

• trace|sum (sum of diagonal of matrix)
• diag|basis|orbitals, diagonal of matrix
• atoms, trace for each atom of the matrix
• matrix, full matrix

Others?