Add DimDict #339
Description
Related to #321 - Dealing with dimensions is painful as you need to worry about the name of each dimension. This makes it difficult as you need to standardize the names of the dimensions or if you are working with multiple OutputVars, you choose one OutputVar and use the naming of those dimensions to standardize. Furthermore, there could potentially be type instability from the number of dimensions not being known unless you use ndims(var.data).
To improve on all these issues, it would be nice to make another struct called DimDict whose purpose is to map dimension name to dimension vectors. This could used tuples or some other container as the keys and values with linear search instead of hashing. Lastly, type stability can be improved as information about the length can be included in the struct.