Comparison of values with Unit should not throw ValueError.

[mrshannon]

The __eq__ method of Unit attempts to convert any value it is compared with to a Unit. The problem is when this fails it should return NotImplemented (or raise NotImplementedError) instead of raising ValueError. Because of this, comparisons such as:

Unit('meter') == {}

raises

ValueError: [UT_UNKNOWN] Failed to parse unit "{}": b'Resource temporarily unavailable'

when it should return False. By returning NotImplemented you can punt the decision to the other object or fall back on object identity (if neither object can handle the comparison).

[bjlittle]

Closed by #150