allow xfield and yfield paramters to be scalar values
Merge pull request #901 from ContinuumIO/feature/xyfield_values

allow xfield and yfield paramters to be scalar values
Subtypes, such as numpy int64, are not considered instance of np.int. Use np.issubdtype instead. Issue exists when trying the crossfilter demo at /scripts/crossfilter.py.
Merge pull request #910 from rothnic/master

Subtypes, such as numpy int64, are not considered instance of np.int.
Merge branch 'master' into feature/tests
add tests for resources.py