morgul pedestal: work on primary data, do not require VDS to merged.h5

i.e. default is to acquire 4x½ module data sets but code still assumes 2x1 module, which we hacked by making a VDS but would be good to have `morgul pedestal` recognise the primary data

Start at the `morgul pedestal` entry point and trace how it selects input data and expects `merged.h5`. Compare the default 4×½-module primary data with the assumed 2×1-module layout. Done means the command recognises primary data directly without requiring a VDS.

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