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.

Closes #4.

*Example*

>> a = dseries(randn(4,5),'2000Q1')

a is a dseries object:

       | Variable_1 | Variable_2 | Variable_3 | Variable_4 | Variable_5
2000Q1 | 2.4245     | -1.036     | -0.87587   | -0.57001   | -1.6989
2000Q2 | 0.9594     | 1.8779     | 0.31995    | -1.0257    | 0.6076
2000Q3 | -0.31577   | 0.9407     | -0.55829   | -0.90875   | -0.1178
2000Q4 | 0.42862    | 0.78735    | -0.31143   | -0.2099    | 0.69916

>> a.Variable_3>0

ans =

  4x1 logical array

   0
   1
   0
   0

>> a(a.Variable_3>0)

ans is a dseries object:

       | Variable_1 | Variable_2 | Variable_3 | Variable_4 | Variable_5
2000Q2 | 0.9594     | 1.8779     | 0.31995    | -1.0257    | 0.6076