Add local_census_records importation model

This model without database allow us to validate incoming file extension and
headers and also does the following during importation process:

* Ignore empty rows
* Classifiy rows in two groups: created_records, invalid_records

app/models/local_census_records/import.rb

@@ -0,0 +1,84 @@
+require "csv"
+
+class LocalCensusRecords::Import
+  include ActiveModel::Model
+
+  ATTRIBUTES = %w[document_type document_number date_of_birth postal_code].freeze
+  ALLOWED_FILE_EXTENSIONS = %w[csv].freeze
+
+  attr_accessor :file, :created_records, :invalid_records
+
+  validates :file, presence: true
+  validate :file_extension, if: -> { @file.present? }
+  validate :file_headers_definition, if: -> { @file.present? && valid_extension? }
+
+  def initialize(attributes = {})
+    if attributes.present?
+      attributes.each do |attr, value|
+        public_send("#{attr}=", value)
+      end
+    end
+    @created_records = []
+    @invalid_records = []
+  end
+
+  def save
+    return false if invalid?
+
+    CSV.open(file.path, headers: true).each do |row|
+      next if empty_row?(row)
+
+      process_row row
+    end
+    true
+  end
+
+  private
+
+    def process_row(row)
+      local_census_record = build_local_census_record(row)
+
+      if local_census_record.invalid?
+        invalid_records << local_census_record
+      else
+        local_census_record.save
+        created_records << local_census_record
+      end
+    end
+
+    def build_local_census_record(row)
+      local_census_record = LocalCensusRecord.new
+      local_census_record.attributes = row.to_hash.slice(*ATTRIBUTES)
+      local_census_record
+    end
+
+    def empty_row?(row)
+      row.all? { |_, cell| cell.nil? }
+    end
+
+    def file_extension
+      return if valid_extension?
+
+      errors.add :file, :extension, valid_extensions: ALLOWED_FILE_EXTENSIONS.join(", ")
+    end
+
+    def fetch_file_headers
+      CSV.open(file.path, &:readline)
+    end
+
+    def file_headers_definition
+      headers = fetch_file_headers
+      return if headers.all? {|header| ATTRIBUTES.include? header } &&
+        ATTRIBUTES.all? {|attr| headers.include? attr }
+
+      errors.add :file, :headers, required_headers: ATTRIBUTES.join(", ")
+    end
+
+    def valid_extension?
+      ALLOWED_FILE_EXTENSIONS.include? extension
+    end
+
+    def extension
+      File.extname(file.original_filename).delete(".")
+    end
+end