test: reference cases for variable, chord-length and tapered fillets (#776)

Four analytic reference cases for variable, to-zero, chord-length and taper-to-fit fillets, with the formulas in tools/oracle-analytic.

tests/oracle/77-fillet-variable-linear/expected.json

@@ -0,0 +1,25 @@
+{
+  "source": "analytic",
+  "method": "box 40x30x20 with its top front edge (y=0, z=20) rounded at a radius running straight from 2 (x=0) to 5 (x=40): sections are quarter rounds, V = abc - (1 - pi/4) L (a^2 + ab + b^2) / 3; moments and the round's area integrated along the edge (tools/oracle-analytic/cases_fillets.py)",
+  "body_count": 1,
+  "volume_mm3": 23888.407044966672,
+  "area_mm2": 5133.743774393379,
+  "center_of_mass": [
+    19.974846161095797,
+    15.065997951540359,
+    9.957359184584975
+  ],
+  "bbox": {
+    "min": [
+      0.0,
+      0.0,
+      0.0
+    ],
+    "max": [
+      40.0,
+      30.0,
+      20.0
+    ]
+  },
+  "faces": 7
+}