Display intervals correctly

[marcodeangelis]

An interval should be displayed so that rounding is inclusive. So for example the following interval: x=Interval(lo=-1/3,hi=1/3) should display as follows:

[-0.33, 0.34]

when rounded to two decimal digits. Currently this is the behaviour of intervals:

x=I(lo=-1/3,hi=1/3)
print(x)
# [-0.3333333333333333,0.3333333333333333]

Another important addition is to give context specific printing instructions, e.g. number of decimal figures.