Disallow mismatching axes units and types

See discussion at https://github.com/ome/ngff/pull/87#discussion_r797433723

Units were introduced in https://github.com/ome/ngff/pull/57 at a SHOULD level of requirement, primarily for extensibility reasons as well as support for axes with arbitrary units. A large part of the discussion about units revolved about the unit validation and the usage of udunits2 as current reference library.

The following axes dictionary is valid according with the current draft version of the specification but is not recommended as the unit is not part of the subset of recommended time units:

    {
        "name": "t",
        "type": "time",
        "unit": "micrometer"
    }

My opinion is that this (potentially corner) case this should be flagged as invalid by the specification.

Using the current specification, I see two immediate ways to enforce this constraint:

• either tighten the constraints around the units available for the space and time axes types i.e. rewrite https://github.com/ome/ngff/blob/18fc37b23cf536d5accacf9a166c8ade7fda2ad8/latest/index.bs#L228-L230 to express that if a unit for the space or time type is specified, it MUST be part of the specified subset
• or add another constraint expressing the relationship between valid udunits and the axes type i.e. disallow a valid time unit to be associated with a space axis (and inversely)

Start with the discussion linked from PR 87 and review how units were introduced in PR 57. Then inspect the referenced latest/index.bs lines 228-230 and compare the two proposed constraint approaches. Done means the specification explicitly disallows mismatching axes units and types.