Expand tags

This function creates a group of scalar tags from a vector tag. For a vector tag named <tag_name> of length N, scalar tags in the form:

```
<tag_name>_000, <tag_name>_001, <tag_name>_002... <tag_name>_N
```

are created and the data is tagged accordingly. This was originally talked about in #235. This is convenient for plotting in VisIt.
Should be a list comprehension.