This commit allows clients to derive an InteHEAD::TimePoint (renamed
from InteHEAD::Date) from a calendrical start point (std::time_t)
and an elapsed time in seconds. This is generally useful for
restart purposes which needs to fill in Y-m-d H:M:S information in
INTEHEAD and has other uses as well. Demonstrate utility in a new
unit test that also shows leap year handling.
While here, also add fractional second resolution to the time-point
handling. INTEHEAD(411) already has microsecond precision and we
should leverage that support.
