Notation change and corresponding code refactor: b bonds to a assets

[rickecon]

We propose changing the notation in the documentation, docstrings, and code the representation of wealth and savings from $b$ to $a$. The current notation uses the character $b$ indicative of "bonds". Examples include $b_{j,s,t}$ and $b_{j,s+1,t+1}$ in the household budget constraint. We propose changing the notation in the documentation and code to $a$ for "assets". Examples of the notation would be $a_{j,s,t}$ for wealth at the beginning of the period and $a_{j,s+1,t+1}$ for the savings decision.

cc: @jdebacker