Integrate async image saver from httomolib into httomo

[yousefmoazzam]

Complete DiamondLightSource/httomolib#9 and merge into httomolib.

It will be helpful to enable asyncio writer for the the cluster benchmarks. I think the idea is to parametrise the on/off switch of asyncio somehow in save_to_images and remove save_to_images_old. We discussed that the parameter could be cluster: True/False for simplicity. By default we can make it False now in the function itself but in yaml_generator we change that parameter to True (just an idea).

[yousefmoazzam]

Fixed by #257

Note that the parameter name asynchronous was chosen in the method in httomolib to be more explicit about the nature of the parameter (ie, users of the method from httomolib may not know why it has a parameter called cluster, but would understand it having a parameter called asynchronous).

If we would prefer to have the parameter name cluster in the httomo pipelines to decide whether the image saver method should run in sync/async mode, a translation of the asynchronous parameter name to the cluster name could probably be added somewhere in httomo without too much difficulty.

See #260 for any follow-ups.