Dimension rename utility

[pp-mo]

There is a complexity here to be sorted out, since data.dimensions.rename(a, b) will inherently break the references within variables' dimension lists.

The code solution should be similar to #69 and #81

[pp-mo]

complexity here to be sorted out, since data.dimensions.rename(a, b) will inherently break the references within variables' dimension lists.

also ... have to watch out for scope masking of dimension names within groups, i.e.. dim "x" within group1 not the same as dim "x" elsewhere