Support for using assigned quantities in external init equations

This release adds support for references to assigned quantities in the initialization equations for external quantities.

Support for C code generation and C-based solvers

• added C code generation for ode and event functions
• added several C based solvers
• added command-line options to examples for simulation parameter control

Array update refactoring

Refacted function interface to update state arrays rather than allocation a new array for each rhs evaluation.

Initial release

0.1

changed back getindex to unsafe mode