merging strands should keep at least one existing name

[dave-doty]

If two strands are merged by adding a crossover or ligating, then at least one of the two strand names should be preserved. If exactly one strand has a name, that should be the name of the new strand.

However currently if exactly one strand has a name, it is erased when merging the strands.

[dave-doty]

More generally, we have the following precedence:

If a property is nullable/optional (e.g., name), then if one strand has the property and the other doesn't, then assign the property from the strand that had it as non-null.

If one strand is a scaffold and the other is not. In that case, no matter which was clicked first, take the properties from the scaffold.

If both strands have the property, then take the new property value from the strand that was clicked (in the case of ligate) or clicked first (in the case of creating a crossover). This means join_two_strands_with_substrands should be re-written to take a Boolean as a paramter properties_from_strand1 indicating whether to take the new properties from strand1 or from strand2.

[dave-doty]

Also re-write nick reducer and delete crossover/loopout/domain reducer to take properties from original strand (e.g., new names are same as name of strand before).