Move `pad_number()` to `strtools`

[ehrenfeu]

Then implement the Examples as unit tests.

Originally posted by @lguerard in #49 (comment)

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

Lines 601 to 623 in f0a1c9c

	def pad_number(index, pad_length=2):
	"""Pad a number with leading zeros to a specified length.
	Parameters
	----------
	index : int or str
	The number to be padded
	pad_length : int, optional
	The total length of the resulting string after padding, by default 2
	Returns
	-------
	str
	The padded number as a string
	Examples
	--------
	>>> pad_number(7)
	'07'
	>>> pad_number(42, 4)
	'0042'
	"""
	return str(index).zfill(pad_length)