[lecture enhancement] solve warnings

[shlff]

Hi @jstac and @mmcky , I find that there are several UserWarning, ComplexWarning and RuntimeWarning in the following lectures (updating):

• arma
• estspec
• opt_tax_recur
• amss

Do you think we should get rid of these warnings? If so, then I would create a PR to solve them.

e.g., (left: RST; right: MYST):

[mmcky]

@shlff I would love PR's to solve these. 👍