Create example_Initialise_JIT_particles_with_initial_values.py

Examples/example_Initialise_JIT_particles_with_initial_values.py

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+### This example shows how to create a particle set in JIT mode while still sampling initial conditions of a variable eg. Temperature
+### This works even with mulitple releases during a simulation
+### This is an example of a solution to the github issue here: https://github.com/OceanParcels/parcels/issues/861
+### This code assumes you have already made the fieldset. (This is not a stand-only piece of code)
+
+# First create a particle class which samples values, in this case it is sampling temperature.
+# Within this class create the variable you wish to sample (temp) and initialise it with a value of 0.
+# Also create a variable (sampled) to identify if it is the particles day of release.
+
+class SampleParticle(JITParticle):         # Define a new particle class
+    sampled = Variable('sampled', dtype = np.float32, initial = 0, to_write=False) # variable to identify if it is just released
+    temp = Variable('temp', dtype=np.float32, initial=0)  # initialise temperature
+
+# Create the kernel to sample temperature
+def SampleTemp(particle, fieldset, time):
+    particle.temp = fieldset.temp[time, particle.depth, particle.lat, particle.lon]
+
+# Create the kernel to get the initial temperature value for each particle.
+# Note at this stage it is calculating the initial values but not recording them in the particle set file
+# This would be useful for calcuating distance travelled etc but not if you want a record of day 0 conditions.
+def SampleInitial(particle, fieldset, time): 
+    if particle.sampled == 0:
+         particle.temp = fieldset.temp[time, particle.depth, particle.lat, particle.lon]
+         particle.sampled = 1
+
+# Create your kernel list to run,
+# Put the SampleInitial Kernel before the advection kernel to get the initial value of temp before the 1st advection step occurs.
+kernels = SampleInitial + pset.Kernel(AdvectionRK4) + SampleTemp