added luksan/ directory, which will eventually contain several routines written by...

added luksan/ directory, which will eventually contain several routines written by Prof. Luksan and made available under the LGPL.  currently, I've converted the PLIN.FOR subroutine, which is a reimplementation of the LBFGS algorithm (and removed the lbfgs/ directory, since the code turns out to be non-free)

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