FileType_HDF5 does not offer a means of overwriting existing datasets
Created by: rks171
The write methods all call preWrite. preWrite will always attempt to create the dataset that the user passed in. If it already exists in the HDF5 file, the HDF5 interface will throw an error. Sometimes we need to overwrite existing datasets (e.g., CTF needs to overwrite pin_powers when CTF_save_simple_restart is called). We can make a new method (overwrite) that does the same thing as 'write' but skips the dataset creation when it already exists, but this will add a lot of stuff to the class. We could also just make 'write' default to overwriting and leave it to the user to check if the dataset already exists if they are worried about overwriting it.