Update platereader.py, incorporating good parts of xml2png.py, then use in xml2png

[sonyahanson]

Update platereader.py, incorporating good parts ofxml2png.py and good parts of current platereader.py:

• Make a constructor with toDataframe(), getWellData(), getColumn(), and getRow()).
• In the process find out whether dataframe or dictionary underneath is easiest for now, and will also see if there's a convenient way to attach the read parameters to the actual data.
• Then use this in new xml2png and ipynbs.

[sonyahanson]

Among the things xml2png can do that should be incorporated in new platereader.py:

• Be able to tolerate xml data with 'OVER's
• Be able to deal with spectra and scans

[jchodera]

What value should OVER be converted to? NaN or Inf maybe?