[#2017] Pass field explicitly to interpolation method

When using the _interp_method function pointer, we have to explicitly
pass in the field as an argument.

Author: fluidnumericsJoe

parcels/field.py

@@ -379,7 +379,7 @@ def _search_indices(self, time: datetime, z, y, x, ei=None, search2D=False):
     def _interpolate(self, time: datetime, z, y, x, ei):
         try:
             bcoords, _ei, tau, ti = self._search_indices(time, z, y, x, ei=ei)
-            val = self._interp_method(ti, _ei, bcoords, tau, time, z, y, x)
+            val = self._interp_method(self, ti, _ei, bcoords, tau, time, z, y, x)
 
             if np.isnan(val):
                 # Detect Out-of-bounds sampling and raise exception