Unclutter epi_archive's print method + add preview of DT

[brookslogan]

Pretty commonly, we'll print an epi_archive x, see a lot but not very useful, then print x$DT to get a better idea what things look like. We should rebalance the print output to include the DT, exclude any redundant information, and move some other information into a summary method. Then we might be able to make DT a private field without much issue, and avoid some worries about aliasing and mutation and key invalidation.

[brookslogan]

Another note from @dsweber2 : we should consider head and tail methods. Probably after #340.

[brookslogan]

Then we might be able to make DT a private field without much issue, and avoid some worries about aliasing and mutation and key invalidation.

Making DT a private field is out of scope. This issue is a blocker to it, though (as is implementing filter for epi_archives).

[brookslogan]

Another note from @dsweber2 : we should consider head and tail methods. Probably after #340.

This is out of scope.