auto_ratio_calibration ignores time-dependent flagging

[PyxieLouStar]

Autocorrelations are averaged in time before being used by cal_auto_ratio.pro to compute auto ratios. This time averaging, performed in vis_extract_autocorr.pro, uses the time_use field from the obs structure to select which time stamps to include. However, the time_use field does not contain any information from time-dependent flagging present in the uvfits file. Thus time stamps which have been flagged in preprocessing (such as by SSINS, for example) are included in the average (and thus also in the final calibration solutions) when they should be excluded.