Add lowest order integration rule for tets (#1130)

README.md

@@ -209,6 +209,13 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html)
 with respect to documented and/or tested features.
 
+### Unreleased
+
+- Added: The lowest order, one point integration rule for tetrahedral elements
+- Changed: Initializing `Basis` for `ElementTetP0` without specifying
+  `intorder` or `quadrature` will now automatically fall back to a one
+  point integration rule
+
 ### [9.1.1] - 2024-04-23
 
 - Fixed: Tests now pass with `numpy==2.0rc1`

skfem/quadrature.py

@@ -80,10 +80,26 @@ def get_quadrature(refdom_or_elem: Union[Type[Refdom], Type[Element], Element],
 def get_quadrature_tet(norder: int) -> Tuple[np.ndarray, np.ndarray]:
     """Return a nth order accurate quadrature rule for the reference
     tetrahedron (0,0,0) (0,0,1) (0,1,0) (1,0,0)."""
-    if norder <= 1:
-        norder = 2
+    if norder < 1:
+        norder = 1
     try:
         return {
+            1: (
+                np.array(
+                    [
+                        [
+                            .25,
+                        ],
+                        [
+                            .25,
+                        ],
+                        [
+                            .25,
+                        ],
+                    ]
+                ),
+                np.array([1 / 6]),
+            ),
             2: (
                 np.array(
                     [