mixed improvements

[schillic]

• added output types to all functions
• turned shorthand function style into function ... end blocks (except for constructors)
• renamed is_contained -> ∈
• checked Add unit tests for d=0 in lazy sets #49
• minor doc fixes

[mforets]

some comments:

• we can have iscontained (or is_contained ?) as an alias.

• some functions enter in 80 characters => they fit nicely as one-liners.. is there any doc-related reason for the change? just aesthetics?

• the Output block of sign_cadlag is wrong, should say: 1 if x≥0, -1 otherwise. shall we update it here? maybe add too:

### Notes

This is the sign function right-continuous at zero (see [cadlag function](https://en.wikipedia.org/wiki/C%C3%A0dl%C3%A0g)). It can be used with vector-valued arguments via the dot operator.

[schillic]

we can have iscontained (or is_contained ?) as an alias.

We already have in, I think this suffices.

some functions enter in 80 characters => they fit nicely as one-liners

Citation needed :) I saw one case in convex_hull_algorithms.jl.
I prefer to break documentation sentences after the period to avoid moving everything around with a later change.

[schillic]

the Output block of sign_cadlag is wrong

I fixed it here.

[mforets]

We already have in, I think this suffices.

ok. it's the same as Base.\in ? or in should be declared too? i didn't find it in this commit

Citation needed :)

i don't have one, no problem :)

[schillic]

We already have in, I think this suffices.

ok. it's the same as Base.\in ? or in should be declared too? i didn't find it in this commit

Sorry, I should have said that. It seems to work automatically. It just aliases ∈ and then calls the right function.