remove unnecessary requirements of mutable objects in many procs

[Vindaar]

Many procs still require a mutable object as their first argument, although in quite a few places that is not strictly necessary.

Needs to be cleaned up and some procs need a better separation between state changing parts and pure parts.

[Vindaar]

One possible solution to this whole problem would be a change of objects to reference objects.

[Vindaar]

In #32 all objects are finally made ref objects. From here only procs which internally call visit_file will require a var argument.
For that proc it's debatable, whether it's required - the H5FileObj is given as a raw pointer to the C library in addition to a Nim callback. We could probably get away with unsafeAddr here too.