Add "exists" method to Filer

[pfeairheller]

The PR adds an exists method to Filer that will check for existence of directory or file before "reopen" creates it.

[SmithSamuelM]

@pfeairheller

I accepted the pull request. But in looking more closely at the code for reopen/remake I think that adding a parameter to reopen/remake like virgin=True/False and some logic changes that there would be a way to make it so you could tell if it ready existed because it would force it to only be opened == True if it was a virgin open (i.e. did not exist before i.e never been opened). But is was going to take more time to figure out than I had so I will come back to it later. This way can also manage the alternate path as well. Created an issue for me to do this later.