Fixed wrong arg order in IMSC attr parsing for cell resolution

[chrsan]

This commit fixes the switched order of columns and rows in the IMSC attribute parsing for cellResolution.

Closes #403

[palemieux]

@chrsan Can you add a unit test at src/test/python/test_imsc_reader.py:

  def test_cell_resolution(self):
    xml_str = """<?xml version="1.0" encoding="UTF-8"?>
    <tt xml:lang="en"
        xmlns="http://www.w3.org/ns/ttml"
        xmlns:ttp="http://www.w3.org/ns/ttml#parameter"
        ttp:cellResolution="32 15">
    </tt>"""
    doc = imsc_reader.to_model(et.ElementTree(et.fromstring(xml_str)))
    self.assertEqual(doc.get_cell_resolution().columns, 32)
    self.assertEqual(doc.get_cell_resolution().rows, 15)

Thanks BTW!

[chrsan]

Thanks for the test case! I've added it in my latest commit.

[chrsan]

@palemieux BTW, I think the initial value for ebutts:linePadding is wrong. It should be 0c and not 1c as it is right now if I'm correct?