Manage ranges check for attributes #25

Description, motivation and use case
The control system must provide a range of acceptable values for writable attributes.

Proposed solution

• Add a valid range to the device access in cases where it is necessary.
• Add a get_range() method, which must return the minimum and maximum values of an attribute, if they exist.

attribute: srmag/ps-corr-sh2/c04-b-ch1/current
range: [-2.0,2.0]
unit: A

attribute: srmag/ps-corr-sh2/c04-b-ch2/current
range: [-2.0,null]  # -2 to + infinity
unit: A