[sort-] do not modify Sheet._ordering #2254

Every sheet instance needs its own _ordering list, which orderBy modifies.
But _ordering is defined at the class level so it can be easily specified per class.
Now we make a copy of the class _ordering when a new sheet is instantiated, so that orderBy doesn't modify the class list.

visidata/sheets.py

@@ -170,6 +170,7 @@ def __init__(self, *names, rows=UNLOADED, **kwargs):
         self.initialCols = kwargs.pop('columns', None) or type(self).columns
         self.resetCols()
 
+        self._ordering = list(type(self)._ordering)  #2254
         self._colorizers = self.classColorizers
         self.recalc()  # set .sheet on columns and start caches