improve bounds argument and bounds_ attribute in ITML

[wdevazelhes]

See comment: #159 (comment)

We should:

• Allow tuples, lists, numpy arrays, (any array like, TODO: see if there is other array-like than these 3 types) as arguments for bounds for ITML
• I think a unique type of return for bounds_ would be better, say a numpy array
• Do tests to ensure that this works as expected
• Do the right advertising in the dosctring