move data from cstwMPC to datasets

[MridulS]

#618

[MridulS]

downstream changes to make:

KrusselSmith REMARK, import datasets

init_infinite in Structural-Estimates-From-Empirical-MPCs-Fagereng-et-al

[MridulS]

Micro-and-Macro-Implications-of-Very-Impatient-HHs DEMARK, import datasets

[MridulS]

This PR makes it possible to import US Survey of Consumer Finances data by

from HARK.datasets import load_SCF_wealth_weights

SCF_wealth, SCF_weights = load_SCF_wealth_weights()

instead of

from HARK.cstwMPC.SetupParamsCSTW import SCF_wealth, SCF_weights

[MridulS]

@sbenthall can you have a look?

[sbenthall]

looks good!