Update PET driver to work with current code (i-pi#334)

* Update PET driver to work with current code

This makes the PET driver compatible with the
current iteration of the PET code. It also
adds some small amount of future-proofing by
allowing to pass through arbitrary keyword arguments.

* Update docstring to be more accurate

drivers/py/pes/pet.py

@@ -26,11 +26,13 @@
 class PET_driver(Dummy_driver):
     def __init__(self, args=None, verbose=False):
         self.error_msg = """
-The PET driver requires specification of a .json model file fitted with 
-    the PET tools, and a template file that describes the chemical makeup of 
-the structure. 
+The PET driver requires (a) a path to the results/experiment_name folder emitted by pet_train
+                        (b) a path to an ase.io.read-able file with a prototype structure
 
-Example: python driver.py -m pet -u -o model.json,template.xyz
+Other arguments to the pet.SingleStructCalculator class can be optionally
+supplied in key=value form after the required arguments.
+
+Example: python driver.py -m pet -u -o "path/to/results/name,template.xyz,device=cuda"
 """
 
         super().__init__(args, verbose)
@@ -43,19 +45,25 @@ def check_arguments(self):
 
         This loads the potential and atoms template in PET
         """
-        arglist = self.args
+        args = self.args
+
+        if len(args) >= 2:
+            self.model_path = args[0]
+            self.template = args[1]
+            kwargs = {}
+            if len(args) > 2:
+                for arg in args[2:]:
+                    key, value = arg.split("=")
+                    kwargs[key] = value
 
-        if len(arglist) == 2:
-            self.model_path = arglist[0]
-            self.template = arglist[1]
         else:
             sys.exit(self.error_msg)
 
         self.template_ase = read(self.template)
         self.template_ase.arrays["forces"] = np.zeros_like(self.template_ase.positions)
         self.pet_calc = PETCalc(
             self.model_path,
-            default_hypers_path=self.model_path + "/default_hypers.yaml",
+            **kwargs,
         )
 
     def __call__(self, cell, pos):