Feature requests: append to an existing file, overwrite existing steps

Created by: kshitij-v-mehta

Feature requests coming from the GTC application:

1. Ability to append data to existing files: If the application crashes and has to restart, it would be good to have the ability to append data to existing files instead of creating new files.

2. Ability to append and overwrite data in existing files: Consider the following scenario. The snapshot data file gets data every timestep, whereas the application writes checkpoint data every few timesteps. Say, the application wrote a checkpoint after 100 timesteps, and it crashed at step 150. At this point, the snapshot file has 150 timesteps. When the application restarts, we resume from timestep 101. Steps 101-150 that exist in the snapshot file must be deleted or overwritten by new data for these timesteps coming in.