[Users] defining a grid function constant in time

[Miguel Zilhão]

hi all,

for the purposes of trying a particular gauge condition, we'd like to have a grid function (let's 
call it "shift_t0") that's set to be equal to the shift at t=0. we don't want this function to 
evolve at all; we just want this grid function to be accessible at all times (with exactly the same 
values).

is there something in particular that we need to watch out for, when doing this? we ask because 
we've noticed that when doing things in a "naive"* way, sometimes NaNs start developing at some 
point in the evolution (seemingly near refinement boundaries and when regriding). we noticed that 
exactly the same thing happens for the "puncture_u" grid function in TwoPunctures: this function is 
only used at t=0; however, if one activates storage for it and asks for its output, at some point 
NaNs start developing in the output, even though the function is never used again. so, what would be 
the standard way of defining a grid function that is constant in time?

thanks,
Miguel & Helvi

* by "naive", we mean essentially the following

in schedule.ccl we have

STORAGE: shift_t0[1]

in interface.ccl we have

CCTK_REAL shift_t0 type=gf timelevels=1 tags='tensortypealias="U" tags='prolongation="none"'
{
   betax_t0 betay_t0 betaz_t0
} "shift vector at t=0"

and we set its value (on the whole grid) at a function call at CCTK_INITIAL after ADMBase_PostInitial