unicode escape character in metadata strings #344

When a unicode string is like u'hi \u2013 was a good year', 2013
is not an acceptable unicode character and may not be encoded. The
exception is now caught with a warning raised that the attribute is
not printable.
multifile variables

Raise a more explicit exception in the case of a variable not being in a URI of a multifile dataset.
Merge branch 'next' into i338-field-objects-for-dataset

Conflicts:
	src/ocgis/api/request/driver/nc.py
	src/ocgis/test/test_ocgis/test_api/test_request/test_driver/test_nc.py