Add a method get_idx() to group

[jinningwang]

Is your feature request related to a problem? Please describe.
N/A

Describe the solution you'd like
A method GroupBase.get_idx(), it returns all existing idxes in a group.

Better cross group indexing for users/AMS

Describe alternatives you've considered
Below method returns an available idx for new devices.

andes/andes/models/group.py

Lines 352 to 356 in 5ab784b

	def get_next_idx(self, idx=None, model_name=None):
	"""
	Get a no-conflict idx for a new device.
	Use the provided ``idx`` if no conflict.
	Generate a new one otherwise.

Additional context
Move following AMS's implementation to ANDES:
https://github.com/CURENT/ams/blob/8bc9504a8281c16236c73decd8e7365b682c706c/ams/models/group.py#L22-L29