[#316] locode: Implement csv OpenFlights airport database

Implement airport database based on csv OpenFlights table. Implement
UN/LOCODE entry matcher. Implement country namespace.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/util/locode/db/airports/db.go

@@ -0,0 +1,76 @@
+package airportsdb
+
+import (
+	"fmt"
+	"os"
+)
+
+// Prm groups the required parameters of the DB's constructor.
+//
+// All values must comply with the requirements imposed on them.
+// Passing incorrect parameter values will result in constructor
+// failure (error or panic depending on the implementation).
+type Prm struct {
+	// Path to OpenFlights Airport csv table.
+	//
+	// Must not be empty.
+	AirportsPath string
+
+	// Path to OpenFlights Countries csv table.
+	//
+	// Must not be empty.
+	CountriesPath string
+}
+
+// DB is a descriptor of the OpenFlights database in csv format.
+//
+// For correct operation, DB must be created
+// using the constructor (New) based on the required parameters
+// and optional components. After successful creation,
+// The DB is immediately ready to work through API.
+type DB struct {
+	airports, countries pathMode
+}
+
+type pathMode struct {
+	path string
+	mode os.FileMode
+}
+
+const invalidPrmValFmt = "invalid parameter %s (%T):%v"
+
+func panicOnPrmValue(n string, v interface{}) {
+	panic(fmt.Sprintf(invalidPrmValFmt, n, v, v))
+}
+
+// New creates a new instance of the DB.
+//
+// Panics if at least one value of the parameters is invalid.
+//
+// The created DB does not require additional
+// initialization and is completely ready for work.
+func New(prm Prm, opts ...Option) *DB {
+	switch {
+	case prm.AirportsPath == "":
+		panicOnPrmValue("AirportsPath", prm.AirportsPath)
+	case prm.CountriesPath == "":
+		panicOnPrmValue("CountriesPath", prm.CountriesPath)
+	}
+
+	o := defaultOpts()
+
+	for i := range opts {
+		opts[i](o)
+	}
+
+	return &DB{
+		airports: pathMode{
+			path: prm.AirportsPath,
+			mode: o.airportMode,
+		},
+		countries: pathMode{
+			path: prm.CountriesPath,
+			mode: o.countryMode,
+		},
+	}
+}