Add conversion routine for annual data.

The routine converts annual data into quarterly data.
Currently two options are implemented:
- equal-per-quarter setting values in each corresponding quarter to
corresponding annual value
- cubic-spline applicable ONLY to flow variables

(cherry picked from commit 53ec34790610f1d8a400b1376876cf1bda3f8395)
(cherry picked from commit cdf249e78121dea324998f0c0e3211221c78682f)

src/utilities/convert/dseriesA2Q.m

+function ts = dseriesA2Q(ds, method)
+
+% INPUTS
+% - ds       [dseries]    annual frequency object with n elements.
+% - method   [char]       1×m array, method used for the time aggregation, possible values are:
+%                            - 'equal-per-quarter',
+%                            - 'cubic-spline' (for flow variables)
+%
+% OUTPUTS
+% - ts       [dseries]    quaterly frequency object with m<n elements.
+
+% Copyright © 2022 Dynare Team
+%
+% This code is free software: you can redistribute it and/or modify
+% it under the terms of the GNU General Public License as published by
+% the Free Software Foundation, either version 3 of the License, or
+% (at your option) any later version.
+%
+% Dynare dates submodule is distributed in the hope that it will be useful,
+% but WITHOUT ANY WARRANTY; without even the implied warranty of
+% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+% GNU General Public License for more details.
+%
+% You should have received a copy of the GNU General Public License
+% along with Dynare.  If not, see <http://www.gnu.org/licenses/>.
+
+if ~isequal(ds.freq,1)
+    error('Conversion only allowed for annual data!')
+end
+
+dq = dates(4, [ds.dates.year(1) 1]):dates(4, [ds.dates.year(end) 4]);
+
+dQ = unique(dq); % dQ.ndat will be the maximum number of periods in ts
+
+tdata = NaN(dQ.ndat, ds.vobs);
+
+switch method
+    case 'equal-per-quarter'
+        counter = 1;
+        for i = 1:4:dQ.ndat
+            tdata(i:i+3,:) = repmat(ds.data(counter,:), [4,1]);
+            counter = counter+1;
+        end
+    case 'cubic-spline'
+        for i = 1:length(ds.name)
+            dAO = firstobservedperiod(ds{ds.name{i}}).year(1):lastobservedperiod(ds{ds.name{i}}).year(1);
+            ids = find(dq.year(:,1)==dAO(1),1);
+            ide = find(dq.year(:,1)==dAO(end),1,'last');
+            qPoints = dAO(1):0.25:dAO(end);
+            tdata(ids+3:ide,i) = spline(dAO, ds{ds.name{i}}.data, qPoints); % First three datapoints are set to NaN
+        end
+    otherwise
+        error('Unknow method.')
+end
+
+ts = dseries(tdata, dQ, ds.name, ds.tex);
+
+end
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