Fixnum and Bignum have been integrated as a single Integer class.

Previously this gave the warning: constant ::Fixnum is deprecated.

https://bugs.ruby-lang.org/issues/12005

This ensures that @system is set only once, avoiding bugs where any change to @url breaks subsequent calls to #system (as was).

Improve Northgate detection by making it more specific, reducing false positives.

westmidlands -> westyorkshire

Added:
Bedford 
Blackpool 
Bracknell Forest 
Calderdale
Cornwall
County Durham
Darlington
Doncaster
East Riding of Yorkshire
Hull
Milton Keynes 
Newcastle upon Tyne
North East Lincolnshire 
North Lincolnshire 
North Somerset 
North Tyneside
Northumberland
Peterborough 
Plymouth 
Poole 
Rutland
Solihull
South Gloucestershire 
South Tyneside
Southend-on-Sea 
Stockton-on-Tees 
Stoke-on-Trent 
Sunderland
Swindon 
Thurrock 
Torbay 
West Berkshire 
Windsor and Maidenhead 
Wolverhampton
York

Added N. Ireland from their centralised site - I haven't added all authorities as it's a long list and I wasn't sure it could cope with it. At the moment only Belfast has applications. 

The councils are listed below and can't be added separately, they all use this site for planning applications.
    Causeway Coast and Glens
    Mid and East Antrim
    Antrim and Newtownabbey
    Belfast
    Lisburn and Castlereagh City Council
    Ards and North Down
    Newry, Mourne and Down
    Armagh, Banbridge and Craigavon
    Mid Ulster
    Fermanagh and Omagh
    Derry City and Strabane District

Added Nottingham

Should we even bother parsing all these fields from the search results pages given that we'll get them on the deeper scrape anyway?

We need to test a group of Idox sites to see how these parameters are used in practice.

This structure allows us to add other missing fields easily if necessary.