Consistent handling of read/write/readwrite metrics

[tilsche]

If a ReadMetric or WriteMetric is requested, and a ReadWritteMetric is already known in the Directory, this should be used.
There should never be multiple Metric objects for the same metric open at the same time in a Directory (see also #3)

[tilsche]

This is addressed in either in the read-write-split or simple-metrics branches. We just need to figure out which to keep.

read-write-split:

• actual class is decided on by config mode during construction
• stored in std::variant
• getting a reference to a specific type is possible when the stored type of the requested type (including subclasses) (e.g. pointers are convertible)
• pro: restrict actual interface on usage
• pro: seems to work, tests successful
• con: my head hurts whenever I think about it - e.g. cannot easily reason whether the code is correct
• con: additional indirection (unique_ptr) is necessary in order to allow move operations of the variant without hurting my head even more

simple-metric:

• checks methods at runtime based on mode it was opened with
• pro: easy to reason about
• con: no static limitation of the used interface

I flavor simple-metric... because every time I thought I figured out the diamond of death it became even worse. How deep is that rabbit hole?

Any other opinions?

[tilsche]

I went with the simple-metric solution a long time ago.