fix #233 - make factories impure (#234)

fixes #233

Pure functions get_activation_by_name and get_optimizer_by_name
construct polymorphic function results.  Removing `pure` enables
building with the LLVM flang-new compiler.

Although Fortran 2003 allowed pure functions with allocatable
polymorphic results, subsequent standards disallowed such functions.
Using this Fortran 2003 feature blocks building with the LLVM and NAG
compilers and likely will block building with future versions of
gfortran once GCC issue 78640 has been fixed.  (See
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78640.)

TODO: A subsequent error still blocks building with NAG, which this
commit does not address.

Author: rouson

src/nf/nf_activation.f90

@@ -733,7 +733,7 @@ pure function eval_3d_celu_prime(self, x) result(res)
   end function eval_3d_celu_prime
 
   ! Utility Functions
-  pure function get_activation_by_name(activation_name) result(res)
+  function get_activation_by_name(activation_name) result(res)
     character(len=*), intent(in) :: activation_name
     class(activation_function), allocatable :: res
 

src/nf/nf_optimizers.f90

@@ -316,7 +316,7 @@ end subroutine minimize_adagrad
 
   ! Utility Functions
   !! Returns the default optimizer corresponding to the provided name
-  pure function get_optimizer_by_name(optimizer_name) result(res)
+  function get_optimizer_by_name(optimizer_name) result(res)
     character(len=*), intent(in) :: optimizer_name
     class(optimizer_base_type), allocatable :: res