Particles: create a field from two dumpreader

[skuschel]

Currently the createField methods are bound to a single dumpreader, that means its possible to create a field with different scalar quantities on each axis. e.g.: plot 'gamma' over 'x'.
It is currently impossible to create a field from multiple dumps, e.g.: 'x in dump 200' over 'x in dump 2'. This requires to pick identical particles from both dump AND return them in the same, but arbitrary order.

• Probably good to find a way to "equalize"(find better word?!) two dumpreader to do exactly that.
• Concept and desired interface needed for creating the plots.

[skuschel]

Demo for matching particles with identical IDs from different dumps is here (remove .txt):
postpic particle matcher.ipynb.txt

[skuschel]

Probably having a new function postpic.particles.createField which takes the form ((ms1, exp1), (ms2, expr2), ....).