Create overwrite/skip option for computations

[spencerahill]

For a specified Calc, check if final output data exists already. If yes and overwrite option not selected, skip it. overwrite provides a switch to toggle this on or off, in case user actually wants to re-calculate and overwrite data.

This could be done on a per-output datatype basis, so if e.g. both reg.av and std data were requested and only the reg.av data exists already, skip it but do write the std data.