Uses the same type of SQL SELECT queries as Translation, which makes
sense given how similar they are.

Tested on test-genome-DBs/homo_sapiens/core, works without errors.

Aborts upon encountering a core database missing the 'rnaproduct' table
but that is in my humble opinion very much desired behaviour, as it could
indicate incomplete application of schema patches in the release this
will be included in.

There is now a dedicated class, Utils::RNAProductTypeMapper, which
provides type_id<->class_name mappings to both RNAProduct constructor
(with the side bonus of no longer having to set the type ID in the
constructor of each subclass) and RNAProductAdaptor. The latter will
likely be used more frequently than the former, therefore the default
map uses type IDs as keys; the reverse map is only generated the first
time it is needed.

In addition to the constructor, the RNAProductTypeMapper.pm defines a
function called mapper() which provides a singleton instance of the
mapper object.

The mappings are hardcoded instead of being retrieved from the database
because the latter could in principle result in execution of arbitrary
code.

Some tests have been defined as well.

1. List of arguments passed to the constructor now references the
superclass (a'la Feature class hierarchy) instead of listing them
explicitly.

2. Arm documentation now correctly lists values this property can take.

At the moment we support microRNA (Bio::EnsEMBL::MicroRNA) and generic
RNA product (Bio::EnsEMBL::RNAProduct). Any other type value will
trigger an exception.

A minimal extension of Bio::EnsEMBL::RNAProduct, with the only new
information being the arm of the hairpin the miRNA has come from. On the
storage side, this information is stored as a new type of rnaproduct
attribute.

The optional argument allows the user to only retrieve RNA products of
specific type, e.g. microRNAs. At the moment the type has to be
specified by a numerical ID, in the future this will be changed to allow
the passing of string codes.

In this particular case we *do* very much want to pass the argument
array to the proper method as it is, both in order to minimise the
overhead of aliases and just in case someone decides in the end to e.g.
to take advantage of direct modification of @_ contents in
get_all_DBEntries().

Thanks, perlcritic.

Needs some extra code in DBEntryAdaptor to actually work.

The former does not work yet because it needs extra code in DBEntryAdaptor.

The former does not work yet because it needs extra code in
AttributeAdaptor.

1. Since, like transcript() and unlike most getters, seq() potentially
triggers database lookups, put all its tests near transcript ones();

2. Make sure everything is prepared so that running test_getter_setter()
on seq() works in local-only mode;

3. Add a test checking for an exception being thrown in the event of
seq() neither having a local sequence nor being able to retrieve one
from the database;

4. Add an online test.

The first one verifies that the internal fetch method in RNAProductAdaptor
has correctly assigned a Transcript to the RNAProduct object it returns.
All transcript() itself does here is return the store assignment.

The second one tests on-demand retrieval of transcripts in transcript()
itself, via an appropriate method in TranscriptAdaptor.