Do not commit PoissonRecon binary
Added method to generate a surface reconstruction for a given assembly.

The assembly is retrieved at the current location, and point cloud and
mesh files are generated using Poisson reconstruction. The source code
needs to be compiled in scripts/surface/PoissonRecon by calling make.
Two additional variables have to be set in settings.py:
SOPNET_POISSONRECON_BIN_DIR: Bin directory of PoissonRecon
SOPNET_SURFACE_DIR: storage folder for mesh files in PLY format

PLY files can be visualized e.g. using Meshlab.