Enhancement to calendar in Lua

[ZhilkinSerg]

What was changed:

• restored get_turn function in calendar class in Lua (returns current turn number) - fixes #24126 (minor regression introduced in #22583);
• exposed several functions in calendar class in Lua;
• added get_time_duration function to Lua for easier creation of time_duration via wrapper functions (TURNS, MINUTES, HOURS and DAYS);
• allow creation of variables of time_point and time_duration classes in Lua;
• slightly updated comments in calendar code.