Change add_metadata! for single samples to set! and insert!

[kescobo]

While writing #74, it occurred to me that the add_metadata!() method I added in #63 for adding to a single sample doesn't really make sense, we should use insert! and set! for that, as done there.

In other words, instead of

add_metadata!(comm, "sample1", Dict(:thing=> "value"); overwrite=false)
add_metadata!(comm, "sample1", Dict(:thing=> "value"); overwrite=true)

We should have

insert!(comm, "sample1", Dict(:thing=> "value"))
set!(comm, "sample1", Dict(:thing=> "value"))

@anikaluo and @annelle-abatoni, this could be a good one for either of you.