restart.sph and outxxx.sph

We have two different routines which write a binary file containing all the information necessary to restart the run at a later time. Subroutine CHKPT writes such a file every NITCH iterations, where the parameter is set in the CHKPT routine. This file, whose format is given in the DUMP subroutine, stores all the important scalar parameters for the run, and then stores all the particle-based quantities, except for the neighbor list information, which can be reconstructed from scratch during a new calculation. The restart.sph file is overwritten each time, to save on memory.

The code is also set to checkpoint every time DTOUT time units pass, writing a file named outxxx.sph, where ``xxx'' is set as a 3-digit integer equal to NOUT, which is iterated every time the file is written. These files are useful for recording and reconstructing the progress of a run later.