Definition of closed sets/open sets

[jarctan]

Add the definitions of closed sets (and of open sets)

[jarctan]

• [ ] Definition of closed sets
• [ ] Proposition 1.2.5 of the textbook that $\emptyset$ and $X$ are closed set
• [ ] Proposition 1.2.5 of the textbook that intersection of closed sets is closed set
• [ ] Union of finite closed sets is closed set