Permit merged columns in configure-generators --spec spec.csv

[tim-band]

In configure-generators there's a --spec column to permit mass settings non-interactively.
The columns in this csv are "table", "column" and "generator".
This issue proposes allowing the second (CSV) column to specify multiple (database table) columns, implying that these columns should be merged before a generator should be assigned.
This would permit null-partitioned grouped generators to be set via this method.

In order to avoid confusion, this list of (database table) columns should not be comma-separated otherwise we will need to use quotes or similar to avoid clashing with the commas of the CSV format, so the suggestion is that this CSV column should be a space separated list of column values, for example:

observation,observation_concept_id observation_unit_id value_as_number value_as_concept value_as_string,null-partitioned grouped_multivariate_lognormal

[andysouth]

Many thanks @tim-band this seems like a good solution to allow us to create a single specification csv that we can apply relatively easily to multiple OMOP projects.