Fix theorem, proposition, section numbering

[shinuito]

There's some commands in amsthm and otherwise to share theorem labelling to definitions and corrorllaries etc.

Similarly there's some commands to turn off section numbers, and set numbering for sections and subsections as if they were sections.

Something like https://tex.stackexchange.com/questions/37189/number-subsections-and-subsubsections-but-not-sections but might require more reading of docs to get it to work properly.

[shinuito]

Some possibly useful links

https://www.overleaf.com/learn/latex/Counters#\counterwithout{somecounter}{anothercounter}

https://en.wikibooks.org/wiki/LaTeX/Counters

https://stackoverflow.com/a/1004231

https://tex.stackexchange.com/questions/37189/number-subsections-and-subsubsections-but-not-sections

this commit also - forgot to reference the issue :)

5052e6a

[shinuito]

\addtocounter{counter}{value} was particularly useful in lecture 17 for manually adjusting numbering when definition 2.2.1 (a) and (b) were separated.