More useful time-depth conversion

[kwinkunks]

This module should be refactored.

It does not tolerate NaNs in the t-d function, eg if you pass a Vp curve with Nans in it.

I'd like to be able to look up times and depths singly.

[kwinkunks]

It might be nice to optionally return the basis too.

And I'm not sure kind='nearest' is the best choice of interpolation.

[kwinkunks]

And... I don't think it works in 3D