Switch to astra and reduce CPU use in deep dive demos

[hrobarts]

In callbacks

• Reduce number of iterations
• Use FGP_TV instead of TotalVariation

In callbacks, preconditioners, htc

• Use astra FBP and projection operators

[lauramurgatroyd]

Don't forget to add yourself as an author!

[hrobarts]

When changing from TotalVariation to FGP_TV, I see some difference in the image metrics for the final example - image metric callbacks with region of interest.

With TotalVariation after 500 iterations

With FGP_TV after 500 iterations

I don't think it's over-fitting to the noise in the data so we don't see the PSNR reduce.

If we want to show this, it may be worth sticking to TotalVariation but just using fewer iterations

[casperdcl]

merged this (c5932e6) in https://github.com/TomographicImaging/training-cloud-jupyterhub/pull/1