fix: validation of element sequences with negative integers

[sr-murthy]

Additional validation required for ContinuedFraction objects constructed from sequences of elements with negative integers - this should only be possible if the first element (a_0 = first quotient) is negative, and negative values for a_1, a_2,... etc should be considered invalid.

# Is valid
>>> ContinuedFraction.from_elements(-1, 2, 3)
ContinuedFraction(-4, 7)

# Should be invalid
>>> ContinuedFraction.from_elements(1, -2, 3)
ContinuedFraction(2, 5)

Changes required in

• continuedfraction.ContinuedFraction.from_elements

Test cases should be updated and/or added.

Docstrings and documentation should be updated as appropriate.