### Introduce new syntax for shifting backward or forward a dynSeries object.

```Suppose that ts is a dynSeries object and let

t = dynTimeIndex();

Then

us = ts.lag();  <=>  us = ts(t-1);

us = ts.lag(2);  <=>  us = ts(t-2);

us = ts.lead();  <=>  us = ts(t+1);
{"ft_lang_label":"__label__en","ft_lang_prob":0.61662424,"math_prob":0.9977103,"size":346,"snap":"2022-27-2022-33","text_gpt3_token_len":98,"char_repetition_ratio":0.16959064,"word_repetition_ratio":0.0,"special_character_ratio":0.3236994,"punctuation_ratio":0.16666667,"nsfw_num_words":0,"has_unicode_error":false,"math_prob_llama3":0.9966469,"pos_list":[0],"im_url_duplicate_count":[null],"WARC_HEADER":"{\"WARC-Type\":\"response\",\"WARC-Date\":\"2022-08-19T23:24:46Z\",\"WARC-Record-ID\":\"<urn:uuid:47d3a111-e501-4079-9608-fb6287869a2c>\",\"Content-Length\":\"172073\",\"Content-Type\":\"application/http; msgtype=response\",\"WARC-Warcinfo-ID\":\"<urn:uuid:ea1985a8-2b28-4fea-9b74-2369f484b384>\",\"WARC-Concurrent-To\":\"<urn:uuid:a466c955-6a0e-4593-8613-0c82893d8b85>\",\"WARC-IP-Address\":\"217.70.191.81\",\"WARC-Target-URI\":\"https://git.dynare.org/JohannesPfeifer/dynare/-/commit/f4c01a43bc2882e7fd18f3cd82344fff9644088c\",\"WARC-Payload-Digest\":\"sha1:JLPRU3GVZ6P75LAZGDCBEY3YV35FVHKI\",\"WARC-Block-Digest\":\"sha1:YSCORXFR7XD2VMY7YK65HMTM7SOAYLAS\",\"WARC-Identified-Payload-Type\":\"text/html\",\"warc_filename\":\"/cc_download/warc_2022/CC-MAIN-2022-33/CC-MAIN-2022-33_segments_1659882573849.97_warc_CC-MAIN-20220819222115-20220820012115-00067.warc.gz\"}"}