This thorn loads stored simulation output to set initial data.

The implementation is very simple - it invokes an external executable (specified
by the 'load_data_exe' parameter) and reads the values from its stdout. See the
popen() call in src/revive.c for arguments passed to the binary. The
implementation this was developed with is [1], but any other executable that
behaves in a compatible way should work.

Example use:
ActiveThorns = "ADMBase ReviveData"

ADMBase::initial_data             = "revivedata"
ADMBase::initial_lapse            = "revivedata"
ADMBase::initial_dtlapse          = "zero"
ADMBase::initial_shift            = "revivedata"
ADMBase::initial_dtshift          = "zero"

ReviveData::load_data_exe = "<...>/revive.py"
ReviveData::simdata_path  = "<path to stored simulation data"
ReviveData::load_time     = 10.0

[1] https://git.khirnov.net/cactus_revive.git/