Add support for International Units (IU)

[PhilPalmer]

Logic

IC50 results from pMN should be expressed in International Units (set by NIBSC/WHO) as this would make the results comparable across different labs. As this can be somewhat tricky to calculate it would also be very useful if this was handled by AutoPlate

Design

The steps to specify a calibrant to automatically generate the IU will be as follows

1. Specify the calibrant wells by types using the new cal type
   

2. Specify the sample_id for the calibrants eg HIC Pool 1
   

3. A new Calibrant tab will appear which can be used to specify the unitage for each calibrant
   

4. AutoPlate will automatically calculate the IU in the downloadable IED (Individual Effective Dose) table containing the international units aka sample_units