normalize missing values

[cmungall]

e.g. Not applicable, missing data, etc

[wdduncan]

Do we to replace with NaNs?

[cmungall]

No, this is pandas-specific. Let's start be looking at GSC guidelines:

https://gensc.org/mixs/

If a value is missing, please consider using the INSDC missing value vocabulary
Not applicable		information is inappropriate to report, can indicate that the standard itself fails to model or represent the information appropriately
Missing	Not collected	information of an expected format was not given because it has not been collected
Not provided	information of an expected format was not given, a value may be given at the later stage
Restricted access	information exists but can not be released openly because of privacy concerns