Add BYM2 to prior distribution options

[trannttoan]

• Add BYM2 prior to prior syntax check
• Add BYM2 prior to effect grouping
• Filter effects for BYM2 prior in prior spec UI
• Add check for BYM2 prior in model spec assertion
• Add BYM2 to formula creation
• Add BYM2 to posterior summary
• Add helper function that construct the ICAR-basis matrix R
• Add BYM2 to Stan-code generation helpers