Add support for specifiying the device that runcircuit is run on as well as the element type

[mtfishman]

The interface is:

using PastaQ
using ITensorGPU

runcircuit([("X", 1)]; device=cu, eltype=Float32)

Internally this transfers the starting state/process and circuit tensors to device and converts the element types of the starting state/process and circuit tensors to eltype.

This also work if you pass the gate tensors and state/process explicitly.

[mtfishman]

Hopefully fixes #274.

[codecov-commenter]

Codecov Report

Merging #282 (2f498b1) into master (6d49251) will decrease coverage by 0.19%.
The diff coverage is 83.93%.

@@            Coverage Diff             @@
##           master     #282      +/-   ##
==========================================
- Coverage   87.57%   87.38%   -0.20%     
==========================================
  Files          22       22              
  Lines        2205     2298      +93     
==========================================
+ Hits         1931     2008      +77     
- Misses        274      290      +16     

Impacted Files	Coverage Δ
src/io.jl	71.95% <0.00%> (ø)
src/measurements.jl	0.00% <ø> (ø)
src/tomography/tensornetwork-processtomography.jl	98.20% <ø> (ø)
src/circuits/trottersuzuki.jl	38.57% <39.02%> (-0.42%)	⬇️
src/itensor.jl	50.00% <40.00%> (+1.06%)	⬆️
src/circuits/gates.jl	78.00% <45.45%> (+0.91%)	⬆️
src/tomography/quantumtomography.jl	80.83% <71.42%> (-0.48%)	⬇️
src/circuits/getsamples.jl	86.11% <82.60%> (-0.94%)	⬇️
src/circuits/noise.jl	83.63% <83.33%> (-1.37%)	⬇️
src/utils.jl	76.74% <87.50%> (+0.32%)	⬆️
... and 15 more

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[mtfishman]

Ok this has been driving me nuts, but apparently if I run the test with the new tests I added ("test_gpu.jl") before I run some of the tests involving process tomography, the process tomography tests fail. I have no clue why that is happening...

If I run the process tomography tests in isolation locally, or run the "test_gpu.jl" tests after those tests, it all passes, so I think I will investigate that later and merge this.