Specify the default dataset for JHelioviewer via a configuration file.

[dgarciabriseno]

Refer to #69 for context

api/src/Database/ImgIndex.php

Lines 1354 to 1358 in a3d9b9d

	if ($verbose) {
	$tree["SDO"]["default"]=true;
	$tree["SDO"]["children"]["AIA"]["default"]=true;
	$tree["SDO"]["children"]["AIA"]["children"]["171"]["default"]=true;
	}

This tree should set a default selection based on a configuration in Config.ini rather than being hard coded.

General steps to fix:

1. Add configuration fields to Config.example.ini. The example config can be set to program SDO, AIA, 304
2. Update the above code to set default fields based on the configuration