Retrieve non-scalar dataset/var attributes

#9 adds funcs to retrieve numeric scalar or text attributes, but not attributes whose value is a vector of numbers or text strings. CF `valid_range` is an example of such that be useful to be able to read.

> I seem to recall C++ supports default arguments; we could look into adding an size_t index=0 or similar argument and pretend all attributes are 1-D arrays (I'm pretty sure netCDF doesn't support 2-D array attributes without a fair bit of wrangling), but that doesn't solve the C interface. Separate functions would be needed there, even if they forward to the same function.

– @DWesl

See thread in https://github.com/bbakernoaa/AMIO/pull/9#discussion_r3980315943

Start with the scalar attribute retrieval functions added in PR #9 and read the linked discussion about C++ defaults and the C interface. Define the vector numeric and text attribute entry points, including the valid_range case, and verify that both interfaces can retrieve them.

Written by the indexing model from the issue text.