Concatenation was failing if the empty object had no frequency set.

Closes #46

Closes: #47

The returned argument is the first input argument.

Conversion to yearly and bi-annual frequencies.

Returned year was wrong for yearly frequency.

The dates2S routine was plain wrong.

H (half year) is still allowed.

Bug introduced in 7bb6a68f when changing the implementation of the
@dates class.

Also:

 - added unit tests,
 - added conversion to bi annual dates.

For all frequencies, counts the number of subperiods since year 0.

First argument, specifying the frequency, can be an integer *or* a character.

This is not necessary since ordering in INFO should not matter, also
this option is not available in Octave 5.x.

The round function in octave does not admit two arguments (the second
being the rounding precision) as in recent versions of Matlab. The same
trick should apply to old Matlab versions, but I do not know when
Mathworks introduced the second argument… A simple solution could be
to always use this trick, and never use the second argument.

This reverts commit 933c51d5.