Interaction list building for a float32 tree does not work

Example: Change the dtype in test_fmm to np.float32, and you will get the following error when trying to compile the code to build the interaction lists:

error: /home/matt/.cache/pocl/kcache/tempfile_zEmpP8.cl:183:29: cannot convert between vector values of different size ('double' and 'coord_vec_t' (aka 'float3'))
error: /home/matt/.cache/pocl/kcache/tempfile_zEmpP8.cl:184:28: cannot convert between vector values of different size ('double' and 'coord_vec_t' (aka 'float3'))

The offending lines are:

        tgt_ext_center = 0.5*(tgt_min + tgt_max);                                                                                                             
        tgt_radii_vec = 0.5*(tgt_max - tgt_min);                                                                                                              

These are found in FROM_SEP_SMALLER_TEMPLATE.