Add utility methods to retrieve the absolute path of the data dir

[AlCalzone]

This PR adds two utility methods to retrieve the absolute path of the data dir. The method included in JS-Controller uses relative paths, which can lead to undesired behavior while developing with npm-linked modules. getAbsoluteInstanceDataDir was added to have a tidy data directory (preferably separate subdirectories by instance).

getAbsoluteDefaultDataDir

const dataDir = utils.getAbsoluteDefaultDataDir();

This returns the absolute path of the data directory for the current host. On linux, this is usually /opt/iobroker/iobroker-data

getAbsoluteInstanceDataDir

// old style
const instanceDataDir = utils.getAbsoluteInstanceDataDir(adapter);
// new style (classes)
const instanceDataDir = utils.getAbsoluteInstanceDataDir(this);

Returns the absolute path of the data directory for the current adapter instance.
On linux, this is usually /opt/iobroker/iobroker-data/<adapterName>.<instanceNr>