py_ext: fix CSV Writer

fix POU logic :
- SAVE is a BOOL
- invocation of py_eval on rising edge of SAVE
- remove save python argument

fix python:
- use no encoding for file open (python2)
- re-use detected dialect if any
- use no "rt+" and truncate since no need to re-sniff dialect for output file
- return "OK" instead of "#SUCCESS", preventing POU logic to ACK result
- support creating new line if writing just after last line
- support appending data on short rows

fix example:
- use a HMI:Button to trigger CSV write instead of HMI:Input +1
- reload CSVs on on each new CSV opened in file browser
- add display of CSV write output

<?xml version='1.0' encoding='utf-8'?>
<PyFile xmlns:xhtml="http://www.w3.org/1999/xhtml" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <variables>
    <variable name="PyVar0" type="DINT"/>
    <variable name="PyVar1" type="DINT"/>
  </variables>
  <globals>
    <xhtml:p><![CDATA[]]></xhtml:p>
  </globals>
  <init>
    <xhtml:p><![CDATA[]]></xhtml:p>
  </init>
  <cleanup>
    <xhtml:p><![CDATA[]]></xhtml:p>
  </cleanup>
  <start>
    <xhtml:p><![CDATA[]]></xhtml:p>
  </start>
  <stop>
    <xhtml:p><![CDATA[]]></xhtml:p>
  </stop>
</PyFile>