Added argument checks to WTSI::NPG::Genotyping::Call::merge.

WTSI::NPG::Genotyping::Fluidgim::AssayResult:

Added is_empty method.

Renamed npg_ to canonical_ in cases where the thing being created is a
canonical version.

Renamed assay_position to assay_address to be consistent with
sample_address.

WTSI::NPG::Genotyping::Fluidgim::AssayResultSet:

Added size method.
Added sample_name method.
Added assay_addresses method.
Added result_at method.
Removed get_calls method.

WTSI::NPG::Genotyping::Fluidgim::Subscriber:

Now both creates and merges calls so that the assay addresses can me
checked across all the resultsets involved.

Modified other code to propagate the API changes.