reimplement postprocessing of SEQ

change https://github.com/mcvine/instruments/blob/ad264eab974c2d74fc76248a510b1f7c7dfdb22e/SEQUOIA/SEQUOIA/beam_postprocessing.py

to sth like
```
from mcvine.instruments import dgs_bpp

LSAMPLE = ???
Ei=???
m2sout = "m2sout"
out = "out.bpp"
dgs_bpp.run(m2sout, out, Ei, LSAMPLE)
```

