remove or deprecate `getGroupSeparator`

[bogovicj]

Could be removed or deprecated. It is used only in deepList methods where its purpose could be replaced by key value access compose or normalize methods.

As a result, one reason this isn't possible now is because not every N5Reader is a KeyValueReader

[cmhulbert]

My understanding was that it was intended to be used to support Windows/Unix path differences. But thats not relevant anymore at the N5Reader level for anything that is backed by a KVA, since we normalize all keys that represent groups to be valid URI paths. (which must have URI separators). FilesystemKVA is a little weird since we do handle paths more flexibly, but I think thats a special case, and even still does not depend on getGroupSeparator.

I'd be for deprecating it, and otherwise ensuring all implementations return '/'