Duplicate/missing example for spectral representations

[jwarner308]

For the spectral representation examples in the stochastic_process module, the N-dimensional & multiple-variables example is the same as the N-dimensional & one-variable example. Therefore, there is not currently an example that demonstrates the N-dimensional & multiple-variables case.

See:
https://github.com/SURGroup/UQpy/blob/master/docs/code/stochastic_processes/spectral/spectral_nd_mv.py

and

https://github.com/SURGroup/UQpy/blob/master/docs/code/stochastic_processes/spectral/spectral_nd_1d.py