Avoid using flat_np_view for value assignment.

[riga]

Right now, we make use of a pattern for in-place value assignment into ak arrays by using a np (view) representation of the flatten ak array.

However, under many conditions, this assignment fails silently, which is dangerous and error prone. For the assignment to work,

• columns types should not contain optionals (e.g. ?float32), and
• the original arrays must not be the immediate result of a masking (which is the case very often).

Therefore, we need to avoid any use of this pattern throughout the framework, which is achieved by this PR.