initial support for loop detection (only zero order forward mode)
Fixed the text of some error messages
additional work on loops: improved sparse forward jacobian (WIP)
additional work on loops: improved sparse forward jacobian (WIP)
added an option to select the AD mode used to generate the Jacobian
made enum values uppercase
additional work on loop creation - jacobian forward mode (WIP)
additional work on loop creation - jacobian
changed some variable names in IndexedDependentLoopInfo
moved non-indexed operations outside loops
initial work on supporting loop generation for the hessian; 
other minor fixes/improvements in the jacobian loop generation
improved test for loop detection;
changed some headers define names in the test folder
improved hessian loop generation
some fixes to the hessian loop generation (added new test cases)
improved linear sections index detection pattern
initial implementation of the reverse second order mode (WIP);
splitted cg_c_lang_compile_model_helper_loops.hpp into several files;
created special OperationNodes for loop forward and reverse operations, loop start and end operations, and index and index assign operations;
created the concept of Index;
loop indexed independents depend on an index operation;
improved/fixed index pattern detection
improved sparse hessian using reverse 2 mode (reduced number of copies) in CLangCompileModelHelper;
sparse jacobian and hessian source code is only generated after reverse and forward methods in CLangCompileModelHelper;
merged the two types of linear index pattern and improved it furtther;
work on the hessian source generation from reverse mode 2 with loops (WIP);
work on the hessian source generation from reverse mode 2 with loops (WIP);
bugfixes related with the source generation for reverse mode 2 with loops (WIP);