Improve convenience constructors for included implementations

[rkurchin]

They're a bit too rigid right now and should ideally be flexible to anything (different types of vectors, tuples, etc.) that might be thrown at them to specify systems.

[cortner]

The way I did it in JuLIP was to have a conversion interface e.g.

_convert_positions(X::Vector) = X 
_convert_positions(X::AbstractVector) = collect(X) 
_convert_positions(X::AbstractMatrix) = # convert into vector of SVectors 

_convert_bc(bc::NTuple{3, Bool}) = bc 
_convert_bc(bc::AbstractVector) = tuple(Bool.(bc)...)

and so forth. So this means that ANY sensible input will be automatically converted into the format needed internally. And it is easy to add new conversion.