Add ICAR model as a prior distribution option

[trannttoan]

• Add ICAR prior to prior syntax check
• Add ICAR prior to effect grouping
• Filter effects for ICAR prior in prior spec UI
• Add check for ICAR prior in model spec assertion
• Add ICAR to formula creation
• Add ICAR to posterior summary
• Add helper functions to constuct neighborhood graphs from geographic units
• Add ICAR to Stan-code generation helpers