Use Mata to process replacements file

This means that Stata will not have to read the replacements file line
by line (pretty slow), and it opens the door to non-csv formats of the
replacements file that are less strictly formatted.

It also paves the way for multiple ID variables and will allow further
validation of the replacements file before replacements begin.
Add stored results
Deprecate option -display-
Check the variable name variable
Check for ID vals in the replacements file but not the dataset in memory
Check storage types

Blank new values ("") are now not allowed for numeric variables.
Remove old code in comments
Merge branch 'mata'