Ligral_v0.2.1

Changes:

• Added Runge Kutta 4th & 2nd order solvers (ode4 and ode2);
• Added realtime simulation;
• Added logging system, which will output debug log when fatal error occurs;
• Modified setting syntax, add nested setting;
• Modified command line parser, move -d parameter logic to doc command;
• Added support for .lig.json file, supported setting syntax and linking syntax;
• Added trace back stack error print out, optimized error print out;
• Added broadcast calculation operands and methods;
• Fixed some bugs.