DI calibration

Direction-Independent (DI) calibration "corrects" raw telescope data. hyperdrive achieves this with "sky model calibration". This can work very well, but relies on two key assumptions:

• The sky model is an accurate reflection of the input data; and
• The input data are not too contaminated (e.g. by radio-frequency interference).

A high-level overview of the steps in di-calibrate are below. Solid lines indicate actions that always happen, dashed lines are optional:

%%{init: {'theme':'dark', 'themeVariables': {'fontsize': 20}}}%%
flowchart TD
    InputData[fa:fa-file Input data files]-->Args
    SkyModel[fa:fa-file Sky-model source-list file]-->Args
    Settings[fa:fa-cog Other settings]-.->Args

    Args[fa:fa-cog User arguments]-->Valid{fa:fa-code Valid?}
    Valid --> cal

    subgraph cal[For all timeblocks]
        Read[fa:fa-code Read a timestep\nof input data]
        Model["fa:fa-code Generate model vis\n (CPU or GPU)"]
        Model-.->WriteModelVis[fa:fa-save Write model visibilities]

        LSQ[fa:fa-code Calibrate via least squares]
        Read-->LSQ
        Model-->LSQ
        LSQ-->|Iterate|LSQ
        LSQ-->Sols[fa:fa-wrench Accumulate\ncalibration solutions]
    end

    cal-->WriteSols[fa:fa-save Write calibration solutions]