BSA — clarification of strand_type and CompositeSeqRegions

The methods

BioSequence.seq_interval(in Interval the_interval),
NucleotideSequence.translate_seq_region(in SeqRegion seq_region, ...)
NucleotideSequence.reverse_complement_interval(in Interval the_interval)

may all be called using a SeqRegion. In this case, the StrandType may be
minus, and if that is the case, the string returned (for seq_interval) or the
string 'transformed' (the other cases) should be taken as
reverse-complemented. In the last case, this will result in a no-op,
i.e. the strand_type leads to reverse-complementing, which is then
reverse-complementing due to the semantics of the method, ending up in the
same string that wouuld be had from seq_interval().

We suggest adding verbiage to explain these semantics (if, of course, everyone
agrees that this is the expected behaviour!)

Similar to this issue is that all CompositeSeqRegions are expected to be
translated in a depth-first traversal, along each node of the tree
represented by the CompositeSeqRegions. This includes those nodes that have
region_operator == JOIN or ORDER.

accepted