modcc: parse and process `LINEAR` blocks (#840)

Add support for parsing and processing `LINEAR` blocks: 

Changes: 
* `SOLVE` expressions can be called from inside an `INITIAL` block, but only if they are solving a linear system
* Tilde expressions can now be either linear expressions or reaction expressions
* Linear expressions need to be rewritten before being sent to the solver, this is done using `LinearRewriter`
* The linear system is setup in `LinearSolverVisitor` fills the lhs and rhs of the symbolic matrix   
* The matrix is recued using `gj_reduce`, which now works on non-diagonal matrices. 

Fixes #839