spacetime_xcor and closepairs interfaces

1. A new function spacetime_xcor that gives spatiotemporal cross-correlations
2. Improved interfaces to closepairs/crosspairs that decide for the user whether to sort on x or t.

There were also some bug fixes, especially in crosspairs, which had hard-to-debug indexing issues when (t2 - t1 == taumax)

Closes #7 (closed) Closes #5 (closed)