Ch 3

[kbjarkefur]

This PR includes a rewrite of Ch3 mainly based on 2 new addtions:

• DIME Data Plan template with the three components - Data Linkage Table, Master Datasets, Data Flow Charts
• The definition of "research variables" and "measurement variables"

[kbjarkefur]

This is now ready for review. There is some stuff in there that we have discussed in meetings but not discussed after seen written. I am looking forward to see what you have to say.

[mariaruth]

First batch of comments -- i'm worried the review feature will bug so submitting now and will finish reviewing the rest of the chapter after.

[mariaruth]

having some trouble with the review function, moving to single comments

[mariaruth]

all set from my side, @luizaandrade a few inputs from you needed before merging