fixed syntax errors and clarify some grammar
more syntax fixes

Apparently, when I edited this lab earlier I probably did an unfortunate
replace all.
reorganized introduction

changed the introduction to focus more on cython being an automation of
writing a C-extension for Python.
edits to typed declarations section of lab

Reword and reorder some of the content for clarity.
center figures
Merge pull request #253 from ayr0/cython_fixes

fixed syntax errors and clarify some grammar