compute default number of checkpoints per file

assume each checkpoint file should be at least order 1GB, and then check
how much space each checkpoint will require.
this includes both field and particle data.

bfps/DNS.py

@@ -655,6 +655,21 @@ class DNS(_code):
             opt.dky = 2. / opt.Ly
         if type(opt.dkx) == type(None):
             opt.dkz = 2. / opt.Lz
+        if type(opt.nx) == type(None):
+            opt.nx = opt.n
+        if type(opt.ny) == type(None):
+            opt.ny = opt.n
+        if type(opt.nz) == type(None):
+            opt.nz = opt.n
+        if type(opt.checkpoints_per_file) == type(None):
+            # hardcoded FFTW complex representation size
+            field_size = 3*(opt.nx+2)*opt.ny*opt.nz*self.fluid_dtype.itemsize
+            checkpoint_size = field_size
+            if self.dns_type == 'NSVEp':
+                particle_size = (1+opt.tracers0_integration_steps)*3*opt.nparticles*8
+                checkpoint_size += particle_size
+            if checkpoint_size < 1e9:
+                opt.checkpoints_per_file = int(1e9 / checkpoint_size)
         self.pars_from_namespace(opt)
         return opt
     def launch(