Excellon format detection uses ExcelonFile.bounds now

Long term we should have only one .bounds method. But ExcellonParser right
now is not correct for cases with two drills in the same line
(it will report one dimension being zero)

gerber/excellon.py

@@ -604,8 +604,8 @@ def detect_excellon_format(data=None, filename=None):
             settings = FileSettings(zeros=zeros, format=fmt)
             try:
                 p = ExcellonParser(settings)
-                p.parse_raw(data)
-                size = tuple([t[0] - t[1] for t in p.bounds])
+                ef = p.parse_raw(data)
+                size = tuple([t[0] - t[1] for t in ef.bounds])
                 hole_area = 0.0
                 for hit in p.hits:
                     tool = hit.tool