Built-in Gravity Grid Options

We have several built-in options for the code, to cover a variety of situations. They are defined in the input files by the parameter ``NGR'', as well as the parameters ``XGRMAX, XGRMIN, XGRLIM, etc.'' They work as follows:

• ${\tt NGR}=0$: No self-gravity at all
• ${\tt NGR}=1$: Fixed gravity grid, running from ${\tt XGRMIN}< x < {\tt XGRMAX}$, ${\tt YGRMIN}< y < {\tt YGRMAX}$, and ${\tt ZGRMIN}< z < {\tt ZGRMAX}$.
• ${\tt NGR}=2$: The gravity grid will be adjusted every timestep to cover all of the particles, no matter how far from the origin or each other.
• ${\tt NGR}=21$: Initially like ${\tt NGR}=2$, but after a certain time ``TGR'' has passed, switch to ${\tt NGR}=1$, with ${\tt -XGRLIM}< x < {\tt XGRLIM}$, ${\tt -YGRLIM}< y < {\tt YGRLIM}$, and ${\tt -ZGRLIM }< z < {\tt ZGRLIM}$.
• $90 \le {\tt NGR} \le 99$: Like ${\tt NGR}=1$ at first, with ${\tt -XGRLIM}< x < {\tt XGRLIM}$, ${\tt -YGRLIM}< y < {\tt YGRLIM}$, and ${\tt -ZGRLIM }< z < {\tt ZGRLIM}$. Sets the grid boundaries to contain ${\tt NGR} \%$ of the total mass of the matter in a given dimension once a particle leaves the range ${\tt -XGRLIM}< x < {\tt XGRLIM}$, etc.
• $290 \le {\tt NGR} \le 299$: Like ${\tt NGR}=2$ until any particle leaves the box defined by ${\tt -XGRLIM}< x < {\tt XGRLIM}$, ${\tt -YGRLIM}< y < {\tt YGRLIM}$, and ${\tt -ZGRLIM }< z < {\tt ZGRLIM}$. From that point onwards, NGR is changed to a new value $({\tt NGR}-200)$.
• ${\tt NGR}=398$: Like ${\tt NGR}=98$, but the grid cannot expand indefinitely. The boundaries are constrained such that ${\tt XGRMIN} \ge -2\times {\tt XGRLIM}$, ${\tt XGRMAX} \le 2\times {\tt XGRLIM}$, etc.