minor refactor of dada reader

now computes integration time t_int from dada headers and a few other things

Signed-off-by: Danny Price <dan@thetelegraphic.com>
More refactoring of dada.py

Did some profiling and found that computing the antisymmetric side of the visibility matrix is expensive.
Have now made computation of this optional, and code is now about 50% faster.

Signed-off-by: Danny Price <dan@thetelegraphic.com>