slight reshuffle of source files

setup.py

@@ -91,15 +91,15 @@ print('This is bfps version ' + VERSION)
 
 ### lists of files and MANIFEST.in
 src_file_list = ['field_descriptor',
-                 'interpolator_base',
                  'rFFTW_distributed_particles',
                  'distributed_particles',
-                 'particles_base',
-                 'interpolator',
                  'particles',
+                 'particles_base',
                  'rFFTW_interpolator',
-                 'fluid_solver_base',
+                 'interpolator',
+                 'interpolator_base',
                  'fluid_solver',
+                 'fluid_solver_base',
                  'fftw_tools',
                  'spline_n1',
                  'spline_n2',