Implement improved visibility format

The RASCIL BlockVisibility format is well-suited to calibration but has the following limitations:

• The lower triangle is needed so the BV is larger than necessary
• The visibility data requires rearranging for imaging: see e.g. nifty gridder
• Conversion to/from MeasurementSets (V2 but maybe also V3) is time-consuming
• Selection by uv coordinates is difficult
• BDA is not straightforward