fix issue with scientific number formats in GCode

The "decimal" module can cause the output of values like "0E-12"
with the "%s" formatting string.

Test case:
   import decimal; print decimal.Decimal("0.0000001")
=> 1E-7

This fix uses a "%.?f" (? = number of digits) format string based
the number of significant digits as given via "minimum step width"
in GCode settings.

pycam/Exporters/GCodeExporter.py

@@ -61,7 +61,8 @@ def _get_num_converter(step_width):
     """
     digits = _get_num_of_significant_digits(step_width)
     format_string = "%%.%df" % digits
-    return lambda number: decimal.Decimal(format_string % number)
+    conv_func = lambda number: decimal.Decimal(format_string % number)
+    return conv_func, format_string
     
 
 class GCodeGenerator(object):
@@ -99,8 +100,8 @@ class GCodeGenerator(object):
                 step_width = minimum_steps[i]
             else:
                 step_width = minimum_steps[-1]
-            conv = _get_num_converter(step_width)
-            self._axes_formatter.append((conv(step_width), conv))
+            conv, format_string = _get_num_converter(step_width)
+            self._axes_formatter.append((conv(step_width), conv, format_string))
         self._finished = False
         if comment:
             self.add_comment(comment)
@@ -302,7 +303,7 @@ class GCodeGenerator(object):
                 continue
             if not self.last_position or \
                     (new_pos[index] != self.last_position[index]):
-                pos_string.append("%s%s" % (axis_spec, new_pos[index]))
+                pos_string.append(axis_spec + self._axes_formatter[index][2] % new_pos[index])
                 self.last_position[index] = new_pos[index]
         if rapid == self.last_rapid:
             prefix = ""