Add test for mathematical operators
Keywords: Add mathmatical operators to the keyword list
Rename the VariableOperation struct to Expression one
Move Expression struct to the separate file
Base implementation of the Expression class
Rename: Expression entity to the Statement one
Functions: Add the string to int conversion functions
Variable: Add typed Get* functions
Variable: Implement the operator =
Initial add the StatementElement struct
Functions: Add the double to string conversion function
Variable: Initial add the operator*
Implement the template string conversion functions
Implement hierarchy of the Variant classes