Fixed a bug in psid_read() where including 1968
variables caused a join failure because ER30001 was dropped
before being used as a join key for the 1968 family file merge.
Fixed a bug in psid_reshape() where the rename step
crashed when ER30001 was absent from the data frame after
psid_read() processing.
psid_str()
for variables names in earlier waves. Thanks Jiao G. for pointing this
out!