Enhance methods of Point returning coordinate values as String

[navispatial]

Goal: Enhance methods of Point returning coordinate values as String
Need from entity classes with point geometries - making json element serialization easier.

Point and Bounds classes, new optional parameter fractionDigits, see method definitions updated:

  /// Writes coordinate values to [buffer] delimited by [delimiter].
  ///
  /// Use [fractionDigits] to set a number of decimals to nums with decimals.
  void writeValues(
    StringSink buffer, {
    String delimiter = ',',
    int? fractionDigits,
  });

  /// Returns coordinate values as a string delimimited by [delimiter].
  ///
  /// Use [fractionDigits] to set a number of decimals to nums with decimals.
  String valuesAsString({
    String delimiter = ',',
    int? fractionDigits,
  });

Point class, new method:

  /// Returns WKT coords (ie. "35 10" for a point with x=35 and y=10).
  /// 
  /// Use [fractionDigits] to set a number of decimals to nums with decimals.
  String toWktCoords({int? fractionDigits}) =>
      valuesAsString(delimiter: ' ', fractionDigits: fractionDigits);

Also internal utility function:

/// Uses `String.toStringAsFixed()` when [n] contains decimals.
/// 
/// For example returns '15' if a double value is 15.00, and '15.50' if a double
/// value is 15.50.
/// 
/// See: https://stackoverflow.com/questions/39958472/dart-numberformat
String toStringAsFixedWhenDecimals(num n, int fractionDigits) =>
    n.toStringAsFixed(n.truncateToDouble() == n ? 0 : fractionDigits);

[navispatial]

Applied on the repository.