Fix usage of deprecated scipy.integrate function

[aidanmontare-fed]

switch from scipy.integrate.simps to scipy.integrate.simpson

scipy.integrate.simps was deprecated in scipy 1.12.0 and will be removed in 1.14 (the upcoming minor version number).

https://docs.scipy.org/doc/scipy/release/1.12.0-notes.html#deprecated-features

This does introduce a requirement of scipy>=1.6.0, which is when scipy.integrate.simpson was introduced

https://docs.scipy.org/doc/scipy/release/1.6.0-notes.html#scipy-integrate-improvements

We could use a conditional import if we want compatability with older versions?

[coveralls]

coverage: 89.396% (-2.7%) from 92.13%
when pulling bb55557 on aidanmontare-fed:fix-deprecated-simpson
into 8faefa9 on aewallin:master.

[aewallin]

scipy 1.6.0 release date seems to be around 2020? I guess the >=1.6.0 requirement doesn't affect that many users..