ENH: Only open auxilliary files for Tipsy as needed

[matthewturk]

Bug report

Bug summary

In yt/frontends/tipsy/io.py around line 160-ish we set up a dict of filehandles for the auxiliary files. These aren't actually necessary to set up unless we want to read from them. That determination happens later on, around line 172.

It might be possible to simple change the iteration for afield in self._aux_fields: to for afield in afields:.