Bug in compute nparticles/tracer/region

Hi,

It seems to me that the mass calculation of marked particles with the compute nparticles/Tracer/Region is not working correct.
The reason therefore may be that the marker of the Particle will be reset by the first call of compute_vector_eval.
As the Method compute_vector calls the compute_vector_eval twice, it have to be ensured that the marker will not be reset by the first call.

The following modification worked for me:
compute_nparticles_tracer_region.cpp 253
replace if(reset_marker_)
by if(countMass && reset_marker_)

BR,
Christoph C.