Improvements on Element value parser and display

Implements multiple suggestions from @lpenasa to improve PTR Element value parser and display:

• Raise an AttributeError on ElementWindow attribute edits:

>>> obs = ObsBlock('2032-01-01T10', '2032-01-01T12')
>>> obs.start = 'foo'
AttributeError: can't set attribute 'start'

• Add xml representation of iterable in Element (separated with double spaces):

>>> Element('foo', [1, 2, 3])
<foo> 1  2  3 </foo>

• Add Element.value type parsing:

>>> Element('foo', [1, 2, 3]).value
[1, 2, 3]

>>> Element('foo', '2032-01-01').value
datetime.datetime(2032, 1, 1, 0, 0)

• Return self on Element.set_value() method:

>>> el = Element('foo', 'bar')
>>> el.set_value('baz')
<foo> baz </foo>

• Fix PTX parser and AGM simulator on non-stripped PTR content:

>>> read_ptr('''
<startTime> 2034-03-20T15:22:25 </startTime>
''')
<startTime> 2034-03-20T15:22:25 </startTime>

Minor changes:

• Customize Element indentation value:

>>> Element.INDENT
'  '

• Update B. Seignovert affiliation.

---

Closes #2 (closed)