TimeType comparisons fail on inf

[shumpohl]

Currently TimeType casts float to TimeType for comparisons. This leads to crashes when doing something like TimeType.from_float(1.1) < float('inf') because infinity cannot be represented as an integer fraction. The error message is not clear because it only says ValueError: invalid digits.

Should we have a max time? somewhing like 1e30ns which is a few orders of magnitude larger than the age of the universe. If one creates a TimeType with a larger absolute value qupulse does no longer garantuee corectness.

• Better error message if a non-finite float(NaN, inf, -inf) is encountered.
• Do not convert other value to TimeType by default for comparisons.