Consider removing dtype from the measurement group configuration

[reszelaz]

This issue is to discuss if we would like to remove dtype from the measurement group configuration as proposed by @tiagocoutinho in https://github.com/sardana-org/sardana/pull/1510#issuecomment-785926629. My first comment about it:

also agree that it does not have too much sense to have the dtype in the expconf (at least I don't know anyone who uses this). I think it is mostly fixed for a detector. I could imagine only detector's dynamic range configuration that could eventually benefit from changing the dtype and in this case, similarly to the shape, the dtype would be a result of the configuration - so again in favor of removing it from the expconf. But, I'm not very experienced in this matter, so better to hear other opinions before we go for this change.

Also, positive reactions were posted by @cpascual and @rhomspuron.

Alternatives are to:

• return dtype, similarly to the shape, using the Controller.GetAxisPar()
• remove this concept completely (but first we should at least see if the currently developed controllers use this information)

[teresanunez]

We do not pass the images to sardana, so we don´t use dtype.