Load monitors separately from the remainder of the files

[SimonHeybrock]

This is a subtask of #82, which is easy to implement in isolation. The required monitors should be loaded independently, without loading the rest of the file.

• Load each monitor on its own, not all together.
• You can assume that all monitors are direct children of NXentry/NXinstrument. The monitor names are defined in the workflow.
• You can assume that position information is self-contained, i.e., there are no depends_on chains leading outside a monitor. We can thus directly use snx.compute_positions on the individual monitor.
• Add a filter to the existing load function such that no monitors are loaded there. This is essential to avoid potential massive overhead from large event-mode monitors.
• The functions for "extracting" monitors for the loaded data probably need to preserved for supporting legacy NeXus files (ISIS). They should be moved to that submodule if feasible.