Added ds_type_resolver function

[RogerJanusiak]

Added a function that allows users to pass in a string or list of strings and get back the dataset object to be used with servicex.

[ArturU043]

Do we want this to be a user-facing function (added in init) or just a helper for internal functions like get_structure() ?

In the second case its probably better to add the prefix _ to the func name. (Actually a chore I have for all current helpers)

This way the user input is further simplified and we might add this feature in the same way to servicex.deliver()

[RogerJanusiak]

@ArturU043 I think that It would be good for it to be user facing. I think this is something that people could use and if enough people use it then we can implement it in the front end directly.

[ArturU043]

Remove unused variable