Add to_h method

[andrew2net]

We store parts of ID in index file. We need to get these pars from pubid with method to_h and recreate PubID from the hash:

> hash = pubid.to_h
{
  code: "800-121",
  publisher: "NIST",
  revision: "2",
  serie: "NIST SP"
}

> Pubid::Nist::Identifier.new(**hash)
 @code="800-121",
 @publisher=#<Pubid::Nist::Publisher:0x0000000109b619d0 @publisher="NIST">,
 @revision="2",
 @serie=#<Pubid::Nist::Serie:0x0000000109b62150 @parsed=nil, @serie="NIST SP">,
 @supplement=nil,
 @update=nil>

[mico]

@andrew2net we already have a method doing the same: #get_params, but #to_h will better reflect purpose of the method, so I'm renaming it.

[andrew2net]

@mico we need to document all the Pubid methods. You can use https://yardoc.org so we automatically get docs like this https://www.rubydoc.info/gems/relaton-bib/1.16.2/RelatonBib/Affiliation

[mico]

When using "typed stages" with input like:

{ type: "tr", number: 1, publisher: "ISO", stage: :dtr }

#to_h returns:

{:number=>1, :publisher=>"ISO", :stage=>#<Pubid::Core::Stage:0x000000010a7e31b8 @config=#<Pubid::Core...>{:long=>"Technical Report", :short=>"TR"}}>, @stages=["40.00"]>>, :type=>"tr", :typed_stage=>"DTR"}

which cannot be used to rebuild identifier again because of typed_stage parameter that we don't accept for constructor, "typed stage" should be applied through stage parameter.

[andrew2net]

@mico can we create Pubid::Core::Stage#to_h method? We need to serialize Pbid as a Hash and store in YAML file. And we need to be able to recreate the Pubid from the Hash. If it's impossible to create the stage parameter in Pubid constructor, let's make Pubid#from_hash parser method.