Refactor Variable class to that you can retrieve specify attribute types

[bena-nasa]

I doing some work and it was helpful to have specific procedures in the Variable class in PFIO to retrieve attributes of specific types. Note there was a helper procedure in FileMetaDataUtilities.F90 that given a FileMetaDataUtilities class, a variable name, and an attribute name you could retrieve an attribute of a specific type but no way, given a Variable to retrieve.

It makes sense to do a simple refactor and move some of this into the Variable class so that this class can have the functionality to get attributes of specific types and the existing interfaces to FileMetadataUtilities can remain unchanged.

I'm just making an issue for the PR I'm going to do as I've already done the refactor.

Will make more sense when one sees the PR.

[weiyuan-jiang]

I am a little bit confused. I think the original get_attribute can just did what you added