Fix splitting behavior when multiple runs of the splitting variable are present

[qitianshi]

See FIXME, line 17:

nano-oscillator/analysis/split.py

Lines 10 to 34 in 8378258

	def __split_variable(
	data: pd.DataFrame,
	var: str,
	reset_t: bool = True
	) -> dict[float, pd.DataFrame]:
	""""Splits data by a variable."""
	#FIXME: This code will group all rows with the same value of var together;
	# expected behavior is that different groups should be separated.
	unique_values = data[var].unique()
	extracted_data = {}
	for phi in unique_values:
	extract = data[data[var] == phi].reset_index(drop=True)
	if reset_t:
	initial_t = extract.at[0, "t"]
	extract["t"] -= initial_t
	extracted_data[phi] = extract
	return extracted_data