Add access methods about soa-derive

[mangelats]

Really useful for the issue #19 where you don't have underlying vectors, so this gives a common interface for both versions and Vec.

[Luthaf]

Sounds good to me!

[mangelats]

Also adding index and index_mut because get actually returns an Option<T>. It will keep the same behaviour as using the index operator:

// vec: Vec<Example>
vec[0];

// soa: ExampleVec
soa.index(0);

Should behave the same (even when it fails) and work with ranges :)