improving readMgfData and scanIndex accessor

 o readMgfData now supports comments and
   PEPMASS with precursor mz and intensity,
   requested by Thomas Taus <2013-02-11 Mon>
 o improved and running read/writeMgfData
   example <2013-02-11 Mon>
 o new scanIndex accessor method <2013-02-11 Mon>

git-svn-id: https://hedgehog.fhcrc.org/bioconductor/trunk/madman/Rpacks/MSnbase@73370 bc3139a8-67e5-0310-9ffc-ced21a209358
Merge branch 'local-hedgehog'