added support for \operatorname (closes matplotlib#553)

I started with the version of the related code written by Freddie
Witherden and committed to (the now abandoned?) mathtex spinoff project
which didn't makes its way back to matplotlib. However, that code:
http://code.google.com/p/mathtex/source/detail?r=75 allowed only to
lower- and upper-case letters to be used inside of \operatorname.

I have improved it to allow the usage of any simple character -
including the insertion of spaces - such that $\operator{arg\,max}$
actually inserts a space between arg max

Author: ivanov

CHANGELOG

@@ -1,3 +1,7 @@
+2011-10-25 added support for \operatorname to mathtext,
+           including the ability to insert spaces, such as
+           $\operatorname{arg\,max}$ - PI
+
 2011-08-18 Change api of Axes.get_tightbbox and add an optional
            keyword parameter *call_axes_locator*. - JJL
 

lib/matplotlib/mathtext.py

@@ -2269,6 +2269,12 @@ def __init__(self):
                     + (group | Error("Expected \overline{value}"))
                     ).setParseAction(self.overline).setName("overline")
 
+        operatorname = Group(
+                        Suppress(Literal(r"\operatorname"))
+                     + ((start_group + ZeroOrMore(simple)  + end_group)
+                        | Error("Expected \operatorname{value}"))
+                     ).setParseAction(self.operatorname).setName("operatorname")
+
         placeable   <<(function
                      ^ (c_over_c | symbol)
                      ^ accent
@@ -2279,6 +2285,7 @@ def __init__(self):
                      ^ genfrac
                      ^ sqrt
                      ^ overline
+                     ^ operatorname
                      )
 
         simple      <<(space
@@ -2578,6 +2585,18 @@ def function(self, s, loc, toks):
         hlist.function_name = toks[0]
         return hlist
 
+    def operatorname(self, s, loc, toks):
+        self.push_state()
+        state = self.get_state()
+        state.font = 'rm'
+        # Change the font of Chars, but leave Kerns alone
+        for c in toks[0]:
+            if isinstance(c,Char):
+                c.font = 'rm'
+                c._update_metrics()
+        self.pop_state()
+        return Hlist(toks[0])
+
     def start_group(self, s, loc, toks):
         self.push_state()
         # Deal with LaTeX-style font tokens