Fix deprecation warning when calling scipy conjugate gradient

[btalamini]

The calls to the CG solver are using a deprecated interface:

DeprecationWarning: scipy.sparse.linalg.cg called without specifying atol. The default value will be changed in a future release. For compatibility, specify a value for atol explicitly, e.g., cg(..., atol=0), or to retain the old behavior cg(..., atol='legacy')