Extend getdataframe and TimestepMatrix to > 2 dimensions

[rjplevin]

I've already ported James' changes to getdataframe > 2 dimensions (in a private fork) but it won't work until we extend TimestepMatrix as well. Since that is likely to undergo changes in the near future, I've bookmarked this here for now...

[rjplevin]

Implemented TimestepArray, and changed TimestepVector and TimestepMatrix to be specializations of this (with number of dimensions, N, bound to 1 or 2, respectively.) I will return to upgrading getdataframe in the near future...

[cledna]

Seconding this! Would be very useful for me.

[davidanthoff]

Ideally we would always use long form here, and extend to more than two dimensions.