[INTERNAL] move xml escape function

- move xml escape function from XMLFile to XMLHandler
- Handler usually has the task to properly escape strings (not the file
  which handles higher level operations)
- [TEST] fix test
[FIX] read and write the filterLine attribute

- fix MzXML file handling, no longer ignore filterLine attribute
[TEST] fix tests
[FIX] add new mzXML attribute: basePeakMz

- [TEST] new test file (where attribute is non-zero as well)
Merge pull request #903 from hroest/fix/mzXML_fix

[FIX] mzXML fix (filterLine, basePeakMz)