Added getNumElements + getVector

[samek]

As reported in issues #3 inserting string vector with the same name doesn't update it but it adds another one.

I've modified inserting so that it checks for the key before adding it to the index.

If this change is not wanted I could maybe create upsertItem upsertItems which would perform this way.

[markkohdev]

Hey @samek thanks for another contribution! You're now a star supporter of this OSS project :) haha

Exposing getNumElements and getVector are 👍

In regards to upserting, after chatting with @dylanrb123 about this change, we don't think that this approach is how we would like to see this implemented. The implementation you have here results in traversal of the entire names list upon every addition in the worst case.

In order to support upserting functionality I think we would need a slightly more involved solution. I'll be implementing StringIndex in the C++ core soon enough and will implement this functionality there (and deprecate this class).

I would recommend removing the upsert functionality for the moment and reducing the scope of this PR. However, if you are very motivated to enable this functionality before StringIndex support in voyager core comes along we can discuss how we might support this using a naive implementation in Java (effectively utilizing parallel HashMaps instead of an ArrayList for managing the strings)

[samek]

Ok, I've removed the functionality.
But I'm keen on making this until we have whole string index in C++ core.
What did you have in mind should we use ConcurrentHashMap ?

[markkohdev]

👍 @samek I'm gonna merge this PR and we can discuss the upsert functionality in #3