Ensure moment derivatives are initialised before they are used

The earlier-initialised values are only used during initialisation to
calculate some advection speeds whose sign is then checked to set
boundary conditions, so the differences caused by this error were small,
but previously (for evolving-upar and evolving-ppar cases) uninitialised
arrays were used, which could change randomly.

src/time_advance.jl

@@ -201,6 +201,12 @@ function setup_time_advance!(pdf, vz, vr, vzeta, vpa, vperp, z, r, vz_spectral,
         fields.phi0 .= fields.phi
     end
 
+    # Preliminary calculation of moment derivatives, to be used for initial version of
+    # 'speed' in advect objects, which are needed for boundary conditions on the
+    # distribution function which is then used to (possibly) re-calculate the moments
+    # after which the initial values of moment derivatives are re-calculated.
+    calculate_moment_derivatives!(moments, scratch[1], scratch_dummy, z, z_spectral, num_diss_params)
+
     ##
     # Charged particle advection only
     ##