Don't import ert in heat-eq forward model

Importing ert increases runtime of ES-MDA from 50s to 2min 30s.

test-data/heat_equation/config.ert

@@ -18,7 +18,7 @@ GRID CASE.EGRID
 
 OBS_CONFIG observations
 
-FIELD COND PARAMETER cond.grdecl INIT_FILES:cond.grdecl FORWARD_INIT:True
+FIELD COND PARAMETER cond.bgrdecl INIT_FILES:cond.bgrdecl FORWARD_INIT:True
 
 GEN_DATA MY_RESPONSE RESULT_FILE:gen_data_%d.out REPORT_STEPS:10,71,132,193,255,316,377,438 INPUT_FORMAT:ASCII
 

test-data/heat_equation/heat_equation.py

@@ -7,10 +7,9 @@
 import geostat
 import numpy as np
 import numpy.typing as npt
+import resfo
 from definition import dx, k_end, k_start, nx, obs_coordinates, obs_times, u_init
 
-from ert.field_utils.grdecl_io import export_grdecl, import_grdecl
-
 
 def heat_equation(
     u: npt.NDArray[np.float64],
@@ -65,9 +64,11 @@ def sample_prior_conductivity(ensemble_size, nx, rng):
     cond = sample_prior_conductivity(ensemble_size=1, nx=nx, rng=rng).reshape(nx, nx)
 
     if iteration == 0:
-        export_grdecl(cond, "cond.grdecl", "COND", binary=False)
+        resfo.write(
+            "cond.bgrdecl", [("COND    ", cond.flatten(order="F").astype(np.float32))]
+        )
     else:
-        cond = import_grdecl("cond.grdecl", "COND", dimensions=(nx, nx))
+        cond = resfo.read("cond.bgrdecl")[0][1].reshape(nx, nx)
 
     # The update may give non-physical parameter values, which here means negative heat conductivity.
     # Setting negative values to a small positive value but not zero because we want to be able to divide by them.