BK: add spending shock

[floswald]

Setup

• a spending shock is a an iid shock that destroys x dollars of liquid asset.
• very important to generate bankruptcies (cue medical spending in the US)
• can be embedded in expectations only: tomorrow's value at asset a' is pi * V(a') + (1-pi) * V(a'-x)
• basically we have a second amount of potential cash on hand tomorrow (right now varies with indices ia and iy, so need to add ix for different spending shock.
• and add another loop here, for example

for iid in 1:p.nD  # future dchoice
    # only feasible choices at this state
    # if renter, cannot sell etc
    if iid == 1
        m1 = m.m1[it+1][iid][jy,:]  # state specific mvec
        c1 = interp(m.c[iid,jy,it+1].env, m1) # C(d',y',m')
        floory!(c1,p.cfloor)   # floor negative consumption
        ctmp[iid,jy,:] = gety(c1)
        # vtmp[iid,jy,:] = vfun(iid,it+1,ctmp[iid,jy,:],m1,m.v[iid,jy,it+1],p)
        vmat[iid,:] += pr * vfun(x->u(x,filer,p),it+1,ctmp[iid,jy,:],m1,m.v[iid,jy,it+1],p)
    else
        # you are a filer: no savings and analytic value function
        ctmp[iid,jy,:] .= income(it+1,p,repeat([m.yvec[jy]],p.na))
        vmat[iid,:] += pr * (u(ctmp[iid,jy,:],true,p) .+ p.beta * bound(m.vbk[jy,it+1]))
        vmat[iid,m.iazero:end] .= -Inf   # filing only with negative assets!
        ctmp[iid,jy,m.iazero:end] .= -Inf   # filing only with negative assets!
    end
end

Assumptions

1. no spending shock while in bk state