Adding Numpy Broadcasting (#222)

* before visualisation

* add visualisation

* summerise rules for broadcasting

* fix exercise and numbering

* fix typos

* fix typos and add examples

* update a sentence

* fix numbers

* fix typos

* fix array format

* fix typos

* update visualisation

* update rules

* fix typos and use concise language

* use more concise descriptions

* fix spaces

* fix punctuations

* wording changes

* restructure the lecture

* change wording

* fix typos

* update based on comments

* update levels

Author: HumphreyYang