Command changes

[thomascobb]

• Rename GetBlockNumbers to GetBlockInfo and make it return Dict[str, BlockInfo] where BlockInfo has number and description
• Rename FieldType to FieldInfo and add description and labels, rename GetFields to GetFieldInfo and make it do the *DESC and *ENUM commands that https://github.com/dls-controls/pymalcolm/blob/b6b5671cce204ed88bb61f9af899e45190591ab9/malcolm/modules/pandablocks/pandablocksclient.py#L206 does

[AlexanderWells-diamond]

This issue is being worked on in PR #25.

This will also obviously include testing for both of these methods, at least one new Example script (mostly so I can test against a real PandA as I go) and probably some more documentation regarding how to introspect a PandA in how-to or explanation format.