Add minimum consumptions c_min_i

[rickecon]

The theory in the OG-Core documentation includes the minimum consumption amounts $c_{min,i}\geq0$ for each consumption good $i$. Including these levels changes the household budget constraint because the minimum consumption values and the tax on those levels must be subtracted from composite consumption. This is not implemented in the code. The current code assumes $c_{min,i}=0$ for all $i$.

cc: @jdebacker