In addition, I've done some fixes to the problem initialization, and I
defined new variables igrids, jgrids, kgrids, which specify the
dimensions of the block.
One array U, which is a field in the BLOCK structure, stores all
variables. The number of variables is determined by the nvars parameter.
To access each variable we use variable index now, like idn, imx, imy,
mz, ien, etc.
A new module for the time integration has been added. This module
contains a set of subroutines to perform one step time integration of
each leaf block using 2nd order Runge-Kutta method. More methods can be
added later. Time 't', timestep 'dt' and iteration 'n' have been moved
to this module as well.
