is_compatible_with ignores active contexts

[keewis]

While working on #1421 I noticed that is_compatible_with allows working with contexts, but they have to be passed explicitly: contexts that were activated using ureg.context (both when used as a function and as a context manager) will be ignored:

In [6]: ureg = pint.UnitRegistry()
   ...: 
   ...: a = 532.0 * ureg.nm
   ...: b = 563.5 * ureg.terahertz
   ...: 
   ...: with ureg.context("sp"):
   ...:     print(a.to(ureg.terahertz))
   ...:     assert a.is_compatible_with(b), "not compatible"
   ...: 
563.5196578947367 terahertz
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-6-8eaa63cfa960> in <module>
      6 with ureg.context("sp"):
      7     print(a.to(ureg.terahertz))
----> 8     assert a.is_compatible_with(b), "not compatible"
      9 

AssertionError: not compatible

(see also #1421 (comment))