Add some backingfile parameter for all functions outputing an FBM

[privefl]

Possibly with NULL option to output a standard R matrix?

[privefl]

As a workaround, I have just added a global option FBM.dir (that defaults to tempdir() as before). This can be used to change the default directory used to create FBMs when calling either FBM(), FBM.code256(), as_FBM(), big_copy(), or big_transpose(). Note that, if not using the temporary directory anymore, you must clean up the files you do not want to keep.

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I am still waiting for someone to implement this, as I think it would not be too difficult.