Fix/empty nodes

[mmolari]

In the code the block_slice function takes care of slicing an alignment on an interval and return a new block with this sub-alignment. Occasionally this can produce an empty entry (when the slice covers a gap in the alignment). What we used to do was then finding all of these empty nodes and removing them afterwards.

I modified the code so that the block_slice function now returns an Option, which is None when a slice is empty. In this way empty nodes should never be produced, and it should not be needed to find and remove them afterwards.