Model partnerships between shipping couriers

Certain shipments (e.g. FedEx SmartPost) are shared between carriers, and until this PR this dataset had no way of modeling that relationship. Previously, consuming libraries would successfully match a FedExSmartPost number like 420112139261290983497923666238 against the FedExSmartPost criteria, and the USPS91 criteria and return both to the consumer. This wasn't wrong, but it wasn't the complete answer either. #12, #23

This change now models both sides of the relationship so that consuming libraries can detect if the tracking number returned is a partnership, and which part of the relationship that tracking number fulfills. One side of the partnership is a shipper (e.g FedEx), and the other side is the [last mile] carrier (e.g. USPS).

This is how it's implemented in the upcoming version of tracking_number

# Search defaults to only showing numbers that fulfill the carrier side of the relationship 
# (if a partnership exists at all), as this is the end a consumer would most likely be interested in.

results = TrackingNumber.search('420112139261290983497923666238') 
=> [#<TrackingNumber::USPS91:0x26ac0 420112139261290983497923666238>]

tn = results.first
tn.shipper? #=> false
tn.carrier? #=> true
tn.partnership? #=> true
tn.partners
 #=> <struct TrackingNumber::Base::PartnerStruct
 #       shipper=#<TrackingNumber::FedExSmartPost:0x30624 420112139261290983497923666238>,
 #       carrier=#<TrackingNumber::USPS91:0x2f1fc 420112139261290983497923666238>>

tn.partners.shipper #=> #<TrackingNumber::FedExSmartPost:0x30624 420112139261290983497923666238>
tn.partners.carrier == tn #=> true

To return both the shipper and carrier when searching, you can pass a second argument into search:

results = TrackingNumber.search('420112139261290983497923666238', match: :all) 
#=> [<TrackingNumber::FedExSmartPost:0x30624 420112139261290983497923666238>, <TrackingNumber::USPS91:0x2f1fc 420112139261290983497923666238>]

TrackingNumbers that are not a partnership fulfill both sides of the relationship and indicate as such:

tn = TrackingNumber.new('1Z879E930346834440')

tn.shipper? #=> true
tn.carrier? #=> true
tn.partnership? #=> false
tn.partners #=> nil

jkeen / tracking_number_data

Model partnerships between shipping couriers #84