title: Pandas Convert 'NA' to NaN
body: <p>I just picked up Pandas to do with some data analysis work in my biology research. Turns out one of the proteins I'm analyzing is called 'NA'.</p>
  
  <p>I have a matrix with pairwise 'HA, M1, M2, NA, NP...' on the column headers, and the same as "row headers" (for the biologists who might read this, I'm working with influenza). </p>
  
  <p>When I import the data into Pandas directly from a CSV file, it reads the "row headers" as 'HA, M1, M2...' and then NA gets read as NaN. Is there any way to stop this? The column headers are fine - 'HA, M1, M2, NA, NP etc...'</p>