Make sample type agnostic and GPU compatible

[tipfom]

Fixed the issue of sample mentioned in #92.

Closes #92.

[mtfishman]

Thanks for the quick PR! This is subtle to test right now, we have to set up the tests in this repository to run on GPUs, I guess we'll address that in future PRs.

[kmp5VT]

@mtfishman I can look around other code in ITensorMPS to see if there is more implicit Float64 typing and open another PR if there is. Thank you @tipfom for your PR!

[tipfom]

Let me know if you want some help with that @kmp5VT. I'd be willing to spend some time on that.

[mtfishman]

Thanks @kmp5VT. Also it would be great to start switching some tests over to being able to test on GPU, looping over device backends the same way we do in the NDTensors/ITensors tests. For now it could just test using JLArrays, but ultimately we should also set things up with Jenkins to run the tests on GPUs.

[mtfishman]

@tipfom I'll merge once you address the comment I made above about changing promote_itensor_eltype to scalartype.

[tipfom]

@mtfishman I've replaced all occurances of promote_itensor_eltype by scalartype. Hope that's okay; sorry for the delay.

[mtfishman]

@mtfishman I've replaced all occurances of promote_itensor_eltype by scalartype. Hope that's okay; sorry for the delay.

Thanks!