Changes v1.9.0 to v2.0.0

• the CasADi namespace is now casadi, #1080
• the directory structure has been cleaned up with the symbolic module now renamed core, cf. #1098 and in-house solvers grouped in a solvers directory, cf. #1136. Imports in examples and source have been changed accordingly, for instance from #include "symbolic/blah.hpp" to #include "casadi/core/blah.hpp" #1051. The central classes are available in C++ via #include <casadi/casadi.hpp>.
• Support for ipopt from apt-get has been implemented (#1012, #1074, #1075, #1077) but is not perfect (#1077, #1102)
• shared libraries are fully supported, static libraries are unsupported (except with MS Visual Studio)
• pkg-config support was added, #424
• solve is no longer an alias for evaluate, #1097
• added the atomic operation fmod, #1112
• We have enforced a CamelCase convention for some class names, meaning that NLPSolver has been renamed NlpSolver, QPSolver has been renamed QpSolver, etc. #1092
• A plug-in design for solvers has been implemented. E.g., instead of creating IPOPT solver with the syntax IpoptSolver(...), you now use a factory implemented in the base class via the syntax NlpSolver("ipopt", ...), cf. #786