🐛 MR set name with dot (`.`) causes parser failure

[slobodan-ilic]

ReadStat fails to parse valid multiple response set strings when the set name contains a dot.

Failing MR string:

Q1.4=D1 1 12 MR set label q1_4_1 q1_4_2 q1_4_3 q1_4_4 q1_4_5 q1_4_6 q1_4_7 q1_4_8 q1_4_9

This is valid per SPSS and PSPP. IBM SPSS opens the file without issue.

Spec reference:
https://www.gnu.org/software/pspp/pspp-dev/html_node/Multiple-Response-Sets-Records.html

Parser likely misinterprets the . in the set name as invalid or as a delimiter.

Expected behavior:
Names with dots (e.g., Q1.4) should be accepted and parsed correctly.

[evanmiller]

Closed via #330