units_convert and conventions

I have tested this compliance checker against a set of data files created by an application I trust.  Three issues came up:
  1. the conventions attribute is CF-1.5
  2. the units of measure for two coordinate variables are convertible to the canonical unit for the standard name 
  3. the units of measure 'hours' were being picked up as a referenced temporal unit (like 'hours since 1970-01-01') which they are not, due to the way cf.util.units_convertible is implemented

I have made a small number of changes to adapt the checker to handle these cases.

This pull request is not ready to merge as there are test failures.  I would appreciate a little feedback on the logic and on whether there are any known test failures to help me with this change proposal.

many thanks
mark
The case I have had a problem with is illustrated by having
```
units1 = 'hours'
units2 = 'hours since 1970-01-01'
```
Whilst
```
  Converter(str(units1), str(units2))
```
does not raise an exception
```
Unit(units1).are_convertible(Unit(units2))
```
returns
```
False
```

These units are not convertible, but the units_convertible function returns True, which has knock on implications for time period coordinates, which do not have references.  The current compliance checker raises errors in these cases, which I am trying to fix.

I am not wedded to the implementation I have proposed, it may well miss other cases.  I need 
```
>>> units_convertible('hours', 'hours since 1970-01-01') 
False
```

Can the behaviour you would like and the behaviour I would like both be delivered?  The code I have proposed could be additional in this function, rather than replacing the original; would that be better?