Improve accuracy of toLatLon

The current implemenation has inaccuracies.  See:
- Turbo87/utm#36
- Turbo87/utm#49

repro:
```
var utm = require("utm");

var originalLatitude = -33.8688
var originalLongitude = 151.2093

const { easting, northing, zoneNum, zoneLetter} = utm.fromLatLon(originalLatitude,originalLongitude);

const { latitude, longitude } = utm.toLatLon(easting, northing, zoneNum, zoneLetter);

console.log(latitude, longitude)
console.log(`lat/lon delta: ${latitude-originalLatitude}:${longitude-originalLongitude}`)

{
  const { easting: easting2, northing: northing2, zoneNum: zoneNum2, zoneLetter: zoneLetter2} = utm.fromLatLon(latitude,longitude);
  console.log(`delta: ${easting - easting2}:${northing-northing2}`);
}
```

index.es.js

@@ -85,7 +85,7 @@ export function toLatLon(easting, northing, zoneNum, zoneLetter, northern, stric
   var n = R / epSinSqrt;
   var r = (1 - E) / epSin;
 
-  var c = _E * pCos * pCos;
+  var c = E_P2 * pCos * pCos;
   var c2 = c * c;
 
   var d = x / (n * K0);