property values are returned as tuple, add append method

[jgrewe]

addresses issues #221, #223, and #227.

The tuple return disallows to append arbitrary types of stuff to the value list. With this we would not need a custom class for this. Internally, it is still stored as a list.

Adding values is possible via the append method which then checks for dtype, etc.
It is also possible to append a property. In this case the values are appended. Implemented the merge function for this.

Please check if you like it. @JuliaSprenger, did you have more specific ideas for issue #221?

[coveralls]

Coverage increased (+0.2%) to 71.103% when pulling 189099c on jgrewe:values into 2830914 on G-Node:master.

[JuliaSprenger]

This looks good, however I noticed some unexpected behaviour.

• When providing a tuple as value this is then stored as a single string element instead of a tuple. This might make sense eg when storing pairs of coordinates, but since the odml values are now also tuples this is a bit confusing.
• Single values are not editable any more. Is there simple a way to edit single values without converting the tuple to a list, editing and setting this as property value?

[jgrewe]

see #237