Need to document PrePARE's checks on variable attributes

[taylor13]

I need to document in https://goo.gl/NmuENr all the PrePARE's checks on variable attributes. Could someone please review the coding starting at line 548 of https://github.com/PCMDI/cmor/blob/master/LibCV/PrePARE/PrePARE.py and tell me for CMIP6:

1. for which variable attributes is an error raised if a required attribute isn't in the file?
2. for which variable attributes is an error raised if the attribute value in the file is incorrect (inconsistent with the data specs)?

thanks.