Change variable names to be Strings

[agisga]

mixed_models expects that all variable names (i.e. column indices in the daru data frame) are ruby Symbols. However, it would make more sense if the variable names were Strings, because Symbols have some restrictions (e.g., they cannot begin with a number) and also that would be more in line with the conventions in daru (for versions >=0.1.1).

[baburdick]

Any string can be symbolized:

:"12345"
<<-TEXT.to_sym
foo
bar
baz
TEXT
# => :"foo\nbar\nbaz\n"