ElegantBook v3.08

Main work

• Fix \part.
• Introduce pad mode.
• Support mtpro2 package.
• Revise default format of references to numbers.
• Include environment for chapter introduction.
• Include environment for problem set.
• Include margin note using \elegantpar (beta).
• Reduce line spacing for equations.
• Turn \equote into \extrainfo, with multi-line display.
• Polish documentation by including acknowledgement, etc.