Add external interfaces

Author: jchristopherson

src/CMakeLists.txt

@@ -16,5 +16,6 @@ set(FSTATS_SOURCES
     ${dir}/distributions_chisquared.f90
     ${dir}/allan.f90
     ${dir}/fstats_errors.f90
+    ${dir}/external.f90
 )
 set(FSTATS_SOURCES ${FSTATS_SOURCES} PARENT_SCOPE)

src/external.f90

@@ -0,0 +1,14 @@
+module external
+
+    interface
+        subroutine SGEMM(transa, transb, m, n, k, alpha, a, lda, b, ldb, beta, &
+            c, ldc)
+            use iso_fortran_env, only : int32, real32
+            character, intent(in) :: transa, transb
+            integer(int32), intent(in) :: m, n, k, lda, ldb, ldc
+            real(real32), intent(in) :: alpha, a(lda,*), b(ldb,*), beta
+            real(real32), intent(inout) :: c(ldc,*)
+        end subroutine
+    end interface
+    
+end module

src/regression_implementation.f90

@@ -1,6 +1,7 @@
 submodule (fstats) regression_implementation
     use linalg
     use fstats_errors
+    use external
     implicit none
 contains
 ! ------------------------------------------------------------------------------