x13: replace NaN by proper missingcode value of x13

Closes #57

See merge request !46

x13: allow for paths with spaces

See merge request !50

x13: allow for Windows paths with spaces

See merge request !49

Avoiding multiple calls when deleting a list of variables in a dseries object.

--style followed by the name of a matlab script (without extension). This matlab
script can be used to apply styling commands to the produced plot.

--title followed by a row char array, used to set the plot title.

x13: save result of function=auto to results if savelog was requested

See merge request !48

lastobservedperiods.m: suppress unit test output

See merge request !47

Accounts for leading missing value. Closes #56

Closes #57

Second output provides the same information but is easier to use (for instance, the second unit test, we use
the new ouput to select variables in a dseries object depending on the last observed period).

Names for dseries obkjects were wrong.

And show that an expression can include scalar variables (defined in
the caller workspace).

Closes #53.

Example:

>> ds1 = dseries(randn(100,3), dates('2000Q1'), {'x','y','z'});
>> ds2 = dseries(randn(100,3), dates('2000Q1'), {'x','y','z'});
>> dplot --expression 2*cumsum(x/y(-1)-1) --dseries toto --dseries noddy --range 2001Q1:2024Q1

This patch completes the change introduced in a519e8a6.

plot.m: fix typo in message

See merge request !44

dummy variable in a dseries object.

plot: do not try to reset xTickLabel based on new dates if hold on was used

See merge request !43

In this version of Octave, “warning on” really enables all warnings (while this
was not the case in Octave 6, suprisingly; I think this used to be different
with even older Octave releases).

It is now supported by the MATLAB editor (as of R2022a).

[skip ci]