Unappended multipliers are allowed

[seanpearsonuk]

>>> t = Q_(1, "h")
>>> t.to("k")
Quantity (1.0, "")

We need to raise an exception.

Also, h is not supported for hour.

[hpohekar]

@seanpearsonuk "h" and "k" are undefined units.

We should not allow construction of undefined units ("h") or unit terms ("h^2 k^-1")

We should raise an exception while constructing these quantities.

We were raising "Not implemented" exception in previous version of quantity. No idea why we have removed this check from Dimension class.

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