Time information in single restart file

[Xiangyu-Hu]

SPHinXsys/src/shared/io_system/io_base.h

Line 154 in c9b66b4

Real readRestartTime(size_t restart_step);

Currently, a separate file is used to save the restart time information.
Also, the each body have their own restart file.
Could we consider add a time entry in single restart xml file, and write all restart state at given iteration step in a single file?