Addresses bug in eventLogViewer, see error message further below.

Is this a bugfix or an enhancement?

bugfix

original error message:

/Users/csoe002/Documents/src/PYME-src/pyme-py37/PYME/DSView/eventLogViewer.py:709: wxPyDeprecationWarning: Call to deprecated item EmptyBitmap. Use :class:`wx.Bitmap` instead
  MemBitmap = wx.EmptyBitmap(s.GetWidth(), s.GetHeight())
Traceback (most recent call last):
  File "/Users/csoe002/Documents/src/PYME-src/pyme-py37/PYME/DSView/eventLogViewer.py", line 716, in OnPaint
    self.DoPaint(MemDC);
  File "/Users/csoe002/Documents/src/PYME-src/pyme-py37/PYME/DSView/eventLogViewer.py", line 632, in DoPaint
    dc.SetTextForeground(self.lineColours[sourceEv])
KeyError: 'ShiftMeasure'

Proposed changes:

As a fix I copied the code from higher up in the eventLogViewer module to the 2nd place where the event source type from the charts is being used as a key:

            if not isinstance(sourceEv, bytes):
                sourceEv = sourceEv.encode()

While this works it feels like a pretty dirty kludge. It almost certainly is a product of the py2 -> py3 transition but it feels like asking for issues at later times. The stored event names in the HDF files are byte type, while the processing in pipeline._processEvents mixes use of byte literals and strings. E.g.

            eventCharts.append(('Focus [um]', zm, b'ProtocolFocus'))

versus

            eventCharts.append(('X Drift [px]', driftx, 'ShiftMeasure'))

This is then used in eventLogViewer and means that some entries are string types while others are byte literals, necessating the code above. Ideally, a more maintainable solution would be nicer, that avoids breakages when code is added/altered. Not sure what the better approach would look like, so looking for suggestions.

Checklist:

• Tested with numpy=1.14
• Tested on python 2.7 and 3.6
• Tested with wx=3.x and wx=4.x [if UI code]
• Does the PR avoid variable renaming in existing code, whitespace changes, and other forms of tidying? [There is a place for code tidying, but it makes reviewing
  much simpler if this is kept separate from functional changes]

If an enhancement (or non-trivial bugfix):

• Has this been discussed in advance (feature request, PR proposal, email, or direct conversation)?
• Does this change how users interact with the software? How will these changes be communicated?
• Does this maintain backwards compatibility with old data?
• Does this change the required dependencies?
• Are there any other side effects of the change?