Updated example_name_spaced_nonlinear_sdof.py to match new eqsig duhamel

[amole-arup]

The problems were that the duhamel library has been deprecated, and the eigenvector solution generates a list.

I have also included the constants (so that note is no longer required), and I have automated the download of the EQ datafile using requests. Neither of the two additional files are therefore required now.