Make steady state masks more user friendly

[Nic2020]

We should make model.sstate.variable.mask[1] = false work instead of using model.sstate.mask and having to find the correct index.

Also, we should probably rename mask with something more informative like solved or known.

[bbejanov]

model.sstate.variable.mask already works - I'm not sure since when, but it's definitely there in the current version (v.0.6.3)

As for renaming mask to something more informative, that'd be nice. Problem is that it requires changes in StateSpaceEcon. It's used a lot in the steady state solver. I'm not sure if it's worth it.