Enhancement: specify alternate directory into which newly trained sigs will be saved

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Would be nice to allow user to choose where newly trained sigs will go, as 
opposed to right beside the tiff by default. The user would specify some other 
top level directory, and the directory structure would be replicated in that 
other directory. Symlinks to tiffs could be created.

The use case is when using different versions of wndcharm to train sigs on the 
same images for feature vector comparison. The naming pattern of the resultant 
sigs is the same.

Perhaps might not be necessary when text-based wndcharm-version-agnostic .sig 
file is replaced by HDF5 file into which wndchrm version is embedded.

Original issue reported on code.google.com by [email protected] on 3 Jan 2013 at 9:31