split up openeo.util

[soxofaan]

openeo.util is becoming a "kitchen-sink" of various unrelated utilities and helpers.

Apart from the general rule of thumb to avoid having a "utils" module (because it often turns into such a hodgepodge over time), it is also not clear which functions/helpers are intended for general usage by users and which are just internal helpers.

We should split it up in more thematic submodules and better flag which helpers are just internal.