Add a description of the group CUSP

Files
file	cusp/cache.cc
	Enter brief description of file here.
file	cusp.cc
	Enter brief description of file here.
file	cusp/delta.cc
	Enter brief description of file here.
file	cusp/local.cc
	Enter brief description of file here.
file	cusp/phi.cc
	Enter brief description of file here.

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Detailed Description