Modify SBML2EML

- use FixedODE1Stepper as continuous Stepper (DE)
- add DiscreteTimeStepper as discrete Stepper (DT)
- Change StepperID of ExpressionAssignmentProcess DE -> DT
- Change Process name for SBML AssignmentRule
- Change Name property of Process delived from SBML Reaction
- Add Name property of Process delived from SBML AssignmentRule
Derive initial value beased on AssignmentRule (incomplete)

Following types of ASTNode are not implemented:
  AST_FUNCTION
  AST_FUNCTION_DELAY
  AST_FUNCTION_PIECEWISE
  AST_LAMBDA
  AST_NAME_TIME
Introduce FunctionDefinitionConverter

User-defined functions are replaced with expanded in-line formula.
minor typo
Introduce Initial Assignment Converter: Not work?
Simplify
Fix CompileGrammar in ExpressionCompiler.cpp

Fixed a problem that ID that starts with “_” was not recognized.
Make VariableReference name as same as Variable ID
Improve ecell3-eml2em

Improve readability of VariableReferenceList
Minor changes: method name etc.
Add some initial value solver components

AST_FUNCTION_DELAY and AST_FUNCTION_PIECEWISE
Change behavior of getInitialValueFromAssignmentRule() and debug around Parameter
Slight arrangement

adjust indent level
Implement piecewise to ExpressionProcesses
Allow to use a Variable repeatedly in a VariableReferenceList
Implementation of delay()
Ref Name in VarRefList for Reaction

Reference name in Reaction’s VariableReferenceList (Param0, Param1, …)
is replaced by Variable Name(ID)
AST_NAME_TIME support: expressed by [[time]] in Expression
[[time]] support
ExpressionEventProcess: incomplete