Document calibrator calibrate field for selective parameter optimization

documentation/rom_simulation/0d-solver/calibrator/readme.md

@@ -67,7 +67,32 @@ Parameter key                        | Description  
 `tolerance_gradient`                 | Gradient tolerance for calibration                                | $10^{-5}$  
 `tolerance_increment`                | Increment tolerance for calibration                               | $10^{-10}$  
 `maximum_iterations`                 | Maximum calibration iterations                                    | 100  
-`calibrate_stenosis_coefficient`     | Toggle whether stenosis coefficient should be calibrated          | True  
-`set_capacitance_to_zero`            | Toggle whether all capacitances should be manually set to zero    | False  
 `initial_damping_factor`             | Initial damping factor for Levenberg-Marquardt optimization       | 1.0  
 
+#### Selecting which parameters to calibrate
+
+Each vessel and multi-outlet junction must declare which of its parameters
+the calibrator should optimize via a `calibrate` field listing the
+parameter names. Parameters that are not listed are held constant at the
+value found in the input file. The names must match the parameter names
+the block exposes (e.g. `R_poiseuille`, `C`, `L`, `stenosis_coefficient`
+for a `BloodVessel`; `R_poiseuille`, `L`, `stenosis_coefficient` per outlet
+for a `BloodVesselJunction`).
+
+```json
+{
+  "vessel_name": "branch0_seg0",
+  "zero_d_element_type": "BloodVessel",
+  "zero_d_element_values": {
+    "R_poiseuille": 0.0,
+    "C": 1.2e-6,
+    "L": 0.25,
+    "stenosis_coefficient": 1.06e-5
+  },
+  "calibrate": ["R_poiseuille"]
+}
+```
+
+If a block has no `calibrate` field (or an empty list), every parameter of
+that block is held at its input value. The calibrator errors out if no
+parameter ends up selected anywhere in the model.