Add "overwrite" option to plugin

[pshriwise]

We don't currently provide a check that the file we're trying to create already exists. For other Cubit commands that create/export a file, an "overwrite" option is provided to clarify that the file should be replaced if the specified filename is already present.

[MicahGale]

If a file conflict is found what's the preferred behavior? Termination with an error? Appending a suffix?

[pshriwise]

I believe the convention is termination with an error as soon as possible, to follow your sentiments in #150.

[gonuke]

It is definitely the Cubit convention to only overwrite when forced with the overwrite keyword, and to fail with an error otherwise. The command terminates, but the in an interactive session , the application doesn't terminate.