`method` selects which, and which arguments are read:

* "eretic" -- electronic reference signal, no reference compound in the
  tube. Needs `analyte_integral`, `eretic_integral`,
  `eretic_response_factor`, `analyte_protons`, `analyte_mw`, and
  `sample_mass_mg` / `sample_volume_ml`. Returns concentration_mm, mass_mg,
  weight_percent.
* "pulcon" -- quantification against a reference measured in a SEPARATE
  tube, via the reciprocity principle. Needs `analyte_integral`,
  `analyte_protons`, `analyte_mw`, `reference_integral`,
  `reference_protons`, `reference_conc_mm`, and optionally the T1, recycle
  delay and field values. Returns concentration_mm, concentration_mg_ml and
  BOTH correction factors (`t1_correction_factor`,
  `field_correction_factor`) -- check them, that is where an over-short
  recycle delay shows up.
* "uncertainty" -- an eight-source GUM budget for a standard internal-
  standard result. Needs `analyte_integral`, `standard_integral`,
  `standard_conc_mm`, `standard_protons`, `analyte_protons`, `analyte_mw`,
  `standard_purity`. Returns the per-source `components`,
  `dominant_source`, `combined_u_rel` and the k=2 expanded uncertainty.

Each method returns a DIFFERENT key set; `method` is echoed back. An
unrecognised `method` returns status "error" naming the three supported.