Add possibility to append to previous timing

[nasailja]

Currently two sets of calls to start+end with the same argument increase the number of calls when calculating calls/s. It would be nice if one could continue the timing of a section i.e. not increase the number of calls when calling start+stop. For example when updating remote neighbor data with dccrg using:

start("update");
start_remote_neighbor_copy_updates();
stop("update");
...
start("update");
wait_remote_neighbor_copy_update_receives();
stop("update");
...
start("update");
wait_remote_neighbor_copy_update_sends();
stop("update");

phiprof would behave as if remote data is updated only once each step. A boolean could be added as an argument to start+stop with default of not appending to previous timing.

[galfthan]

Ok, I'll add this feature.