Reading in initial conditions

[jimc101]

At the moment, particle initial positions are read in from an ascii file. They are several advantages associated within switching to reading in initial positions from a NetCDF file. These include a) variables (e.g. components of the position vector) being self describing, and b) ease with which the file can be arbitrarily extended to include other particle variables (e.g. mass) and properties (e.g. sinking velocity).