Fix nasty bug in data iteration

A plot 'ycalc' can change the cache pointer and upset iterating over the
cache, causing an infinite loop. Using a separate cache for the slice
iterator avoids the problem.

Signed-off-by: Jim Easterbrook <jim@jim-easterbrook.me.uk>

src/pywws/__init__.py

@@ -1,3 +1,3 @@
-__version__ = '19.7.0'
-_release = '1660'
-_commit = '3a926ed'
+__version__ = '19.7.1'
+_release = '1661'
+_commit = '05eab44'

src/pywws/filedata.py

@@ -185,8 +185,10 @@ def _get_slice(self, i):
         a, b = self._slice(i)
         if a > b:
             return
+        # use separate cache as something might change self._rd_cache
+        # during yield
+        cache = _Cache()
         # go to start of slice
-        cache = self._rd_cache
         start, exact = self._get_cache_ptr(cache, a)
         # iterate over complete caches
         while cache.hi <= b.date():