Avoid redefining Python builtins

[DimitriPapadopoulos]

Both complex and input are Python builtin functions.

Note that complex → cplex is consistent with this class definition:

spec2nii/spec2nii/fileiobase.py

Lines 97 to 106 in fee3141

	class unit_conversion():
	"""
	Provides methods to convert between common NMR units
	Parameters
	----------
	size : int
	Number of points in dimension (R|I).
	cplex : bool
	True if dimension is complex, False is real.