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 Dynare/dseries!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 Dynare/dseries!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]

load_m_file_data.m: pass structure instead of file

See merge request Dynare/dseries!41

Next attempt to prevent horse race conditions, see Dynare/dseries!40

load_m_file_data.m: prevent horse-race condition when saving and loading mat file

See merge request Dynare/dseries!40

Add ability to use self-compiled x13as on Mac

See merge request Dynare/dseries!39

Note that conversion to lower frequencies are not implemented since months,
quarters and semesters are not defined in the ISO specification.

Also add unit tests.