Add unique constraints on `(v_astra, spectrum_pk)` to all pipeline tables

[andycasey]

If we switch to using v_astra_major, v_astra_minor, v_astra_patch, -- which I think it should -- then we should just require constraint on v_astra_major.

[andycasey]

Fixed by creating a PipelineOutputModel class, which all pipelines will inherit from. The PipelineOutputModel class does the following:

• stores v_astra as an integer (not a string): major * 1e6 + minor * 1e3 + patch;
• has an auto-generated column (v_astra_major_minor) that is hidden (won't appear in files), evaluated as v_astra_major_minor = v_astra / 1000 and is stored on the PostgreSQL table;
• adds the unique constraint (v_astra_major_minor, spectrum_pk)
• adds some other fields used by all pipeline output tables: task_pk, source_pk, spectrum_pk, v_astra, created, modified, t_elapsed, t_overhead, tag.