[fitsbits] TDIMn

It's worth noting, in case some of you are not aware of this, that
the X-ray community has been using the variable length array
convention since the mid 1990s, and in particular the standard
response matrix format (a basic calibration file produced by
all missions) uses it. So our software almost all supports the heap
convention.
We also have used the TDIMn convention although not that widely,
and I don't think we have used the substring convention anywhere
in our data.
- Jonathan McDowell, Chandra X-ray Center