Remove need for running in prior mode pre-5/7

[markjrieke]

Part of 2.0, but currently the model runs in prior mode from 5/1 -> 5/7. This is just because the fixed effect variables only have one factor for a bit (e.g., only LV polls). The solution is to change the fixed effect variables to not have variable lengths & just map the ids in directly.

[markjrieke]

For example:

• LV = 1
• RV = 2
• V = 3
• A = 4

& g_ref = 1.

And

• DEM = 1
• REP = 2
• None = 3

& c_ref = 3

[markjrieke]

Fixed group // candidate sponsored issue, but still requires at least one observation, so prior mode is set on 5/1 & 5/2. This is fine