HdrSeq: support for a merge() method

For GenShen's RS scan instrumentation, we want support for a merge() method for HdrSeq:s.

The rough shape of the spec for the proposed merge() method in HdrSeq class is:

// Merge the data in this HdrSeq into the HdrSeq, seq2, in the argument
// to the method, optionally clearing the data in this HdrSeq.
void merge(HdrSeq* seq2);

This will likely necessitate having the same or similar methods in
superclass hierarchy.

A PR will be opened once the changes have been implemented, refined,
tested and ready to initial discussion. Opening this issue in anticipation.