#270 Implement XML import for TimeSeries

With a rigorously defined XML format this should be
quite feasible and very useful/popular.

Suggest that dates must match ISO8601 (e.g. yy-mm-dd),
and that multiple series' data may be recorded with
each time point.

Simple suggested sample:
<?xml version="1.0" encoding="utf-8"?>
<chart>
<point>
<date>2004-12-01</date>
<series id="1">8.48</series>
<series id="2">8.55</series>
<series id="3">8.63</series>
<series id="4">105000.0</series>
<series id="5">0.0</series>
<series id="6">10000.0</series>
</point>
...
</chart>

Further details (e.g. about the series) may be recorded
in a section at the beginning of the file.