Add ability to read old-style metadata to v1.5

Old style metadata (`Value` object) should be readable in the new library. The new file format should be upgraded to `1.2.0`. Library-file compatibility would then be as follows:
- NIX `1.4.x` will be unable to read `1.2.0` files
- NIX `1.5.x` will be able to read and write `1.2.0` files
- NIX `1.5.x` will be be able to read (not write) `1.1.1` files

Multivalue properties in `1.1.1` (old) files should be returned as a vector of raw values. The `uncertainty` property should be accessible through a separate method. In case the `Value` objects in the file have different values for uncertainty, a warning should be raised.

Start by locating the metadata reader and the handling for the `Value` object and file-format versions `1.1.1` and `1.2.0`. Use the compatibility matrix, raw vectors for multivalue properties, separate uncertainty access, and warning behavior as the acceptance criteria.

Written by the indexing model from the issue text.