Added SVAR option to plot_cum_effects in irf

[ikuzmin404]

plot_cum_effects function lacked SVAR check: when it was called for SVAR, it returned VAR cumulative IRFs despide them being calculated for SVAR internally. Thus one elif case has beed added to resolve this issue.

Example:

data_d_reer = pd.read_csv(r'data_
[Uploading data_reer.csv…]()
reer.csv', index_col=0)
svar_model = SVAR(data_d_reer, svar_type="A", A=amat, freq='Q').fit()
svar_model.irf().plot_cum_effects();

Current behavior: returns VAR cumulative IR's (self.cum_effects).
Expected behavior: returns SVAR cumulative IR's (self.svar_cum_effects)

[ikuzmin404]

Sorry, for some reason data failed to attach:

data_reer.csv

[pep8speaks]

Hello @ikuzmin404! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file statsmodels/tsa/vector_ar/irf.py:

Line 211:1: W293 blank line contains whitespace