Add Satterthwaite and Kenward-Roger fixed-effect tests

include/spectra_reml/distributions.hpp

@@ -0,0 +1,13 @@
+#pragma once
+
+namespace spectra::reml {
+
+// Two-sided Student-t tail probability and upper F tail probability.
+// Both accept non-integer positive degrees of freedom.
+[[nodiscard]] double student_t_two_sided_p(double statistic,
+                                           double degrees_of_freedom);
+[[nodiscard]] double f_upper_tail(double statistic,
+                                  double numerator_degrees_of_freedom,
+                                  double denominator_degrees_of_freedom);
+
+}  // namespace spectra::reml