```Lagrangian``` with shell mode

[HerculeWu]

Dear Long Wang,

When I use the python data analysis tool, I found one issue about lagrangian.

In the class Lagrangian (defined in PeTar/analysis/lagrangian.py), when use calcOneSnapshot with shell mode, the variabe nlagr (line 393, 394) denotes number of particles between two neighbor Lagrangian radii. Then n_offset (line 435) works not properly. Maybe this issue can be fixed by changing line 435 to

if shell_mode:
    n_offset = np.append(np.zeros(1), rindex+1).astype(int)
else:
    n_offset = np.append(np.zeros(1),nlagr).astype(int)

Thanks and with best wishes, Wu

[lwang-astro]

Thank you for pointing out this bug. Indeed the usage of nlagr to determing n_offset is not correct. It should be rindex+1. I have modified the line 435 to n_offset = np.append(np.zeros(1), rindex+1).astype(int).