soundManager.getOutputLevelForCategory always returns undefined

[pixelzoom]

Noted while working on phetsims/fourier-making-waves#55.

soundManager.getOutputLevelForCategory always returns undefined. It's also missing a @returns annotation.

Here's the current implementation:

  /**
   * get the output level for the specified sound generator category
   * @param {String} categoryName - name of category to which this invocation applies
   * @public
   */
  getOutputLevelForCategory( categoryName ) {

    // Check if initialization has been done.  This is not an assertion because the sound manager may not be
    // initialized if sound is not enabled for the sim.
    if ( !this.initialized ) {
      console.warn( 'an attempt was made to get the output level for a sound category on an uninitialized sound manager, returning 0' );
      return 0;
    }

    // verify that the specified category exists
    assert && assert( this.gainNodesForCategories[ categoryName ], `no category with name = ${categoryName}` );

    return this.gainNodesForCategories[ categoryName ].value;
  }

[pixelzoom]

Done in the above commits. Note that I also did a little doc cleanup, and factored out:

const gainNode = this.gainNodesForCategories[ categoryName ];

@jbphet please review.

[jbphet]

Looks good to me, thanks @pixelzoom. Closing.