Add convention for (numeric) type annotations in function signatures

[mforets]

ref: JuliaReach/LazySets.jl#2276

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Draft:

1. function vertices_list(H::AbstractHyperrectangle{N}) where {N<:Real} . here N is not used in the body of the function => we can remove it from the signature

2. function constraints_list(H::AbstractHyperrectangle{N}) where {N<:Real} here N is used to preallocate outputs, constraints = Vector{LinearConstraint{N, SingleEntryVector{N}}}(undef, 2*n) so we keep where {N}

3. binary operations.... 3i) do nothing if N is not used in the body of the function; remove the type parameter N in both arguments
   ... 3ii) use promote_type(N, M) if it's used in the function

4. inner functions where we already know that the arguments are of the same numeric type. for example, _abs_sum , since rho already makes the conversion.