Improve docstring for `calculate_mean_and_stdv()`

[ehrenfeu]

The docstring should explain in which situations the function will return 0, 0 as a result, i.e. which inputs for values_list will lead to this.

Originally posted by @ehrenfeu in #49 (comment)

python-imcflibs/src/imcflibs/imagej/misc.py

Lines 104 to 128 in f0a1c9c

	def calculate_mean_and_stdv(values_list, round_decimals=0):
	"""Calculate mean and standard deviation from a list of floats.
	Parameters
	----------
	values_list : list of int,float
	List containing numbers.
	round_decimals : int, optional
	Rounding decimal to use for the result, by default 0
	Returns
	-------
	tuple of (float, float)
	Mean and standard deviation of the input list.
	"""
	filtered_list = [x for x in values_list if x is not None]
	if not filtered_list:
	return 0, 0
	mean = round(sum(filtered_list) / len(filtered_list), round_decimals)
	variance = sum((x - mean) ** 2 for x in filtered_list) / len(filtered_list)
	std_dev = round(variance ** 0.5, round_decimals)
	return mean, std_dev