Changes:
	- fix xupdate support for admm
	- fix compilation issues with cuda 6.5
	- implement memory caching for cuda
	- create vec_iter_s for iterative algorithms
	- prepare operator code for n-ary operators (and re-implement operator_p_s)
	- clean-up headers and reorganize lsqr
	- clean-up sense configuration
	- reorganize code (create linops/iter modules)
	- extend md_(z)softthresh API
	- improve md_fill on GPU
	- change TV operator to store partial derivative in new dimension
	- use type checking in operator_apply2
	- change operator interface to allow different number of i/o dims
	- other minor changes