Fixed broken promise chain

[matteopic]

This fixes #60.

The message reported happens because the Svx3dLoader parses the .3d files in two steps; first calling method parse(), then calling parse2().

Survey class performs the following check after the model has been loaded:

if ( surveyData.limits.isEmpty() ) throw new Error( 'Empty survey or region of survey' );

If the model has an init value equal to epsg:27700 the loading works correctly because there is a piece of sync code that handle it, but if it is different then a fetch to https://epsg.io site starts.

...
switch ( init ) {
    case 'epsg:27700' :
sourceCRS = '+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +datum=OSGB36 +units=m +no_defs';
break;

default:
    code = init.match( /(epsg|esri):([0-9]+)/ );
    if ( code !== null ) {
            console.log( `looking up CRS code EPSG: ${code[ 2 ]}` );
            return fetch( `https://epsg.io/${code[ 2 ]}.proj4` )
...

The absence of return inside the Svx3dLoader.load cause the promise chain to break and when the async call to https://epsg.io begin, the execution continues even though the model has not fully loaded.

load ( loadingContext, progress, surveyDataCollector ) {
	return super.load( 'arraybuffer', loadingContext, progress ).then( results => {
                    // Promise chain breaks here
		this.parse( surveyDataCollector, results.data, results.metadata, loadingContext.section, progress );
	} );
}