Group files checker checks for `=` only in first formula in LONG section

Created by: TomTheBear

The group file checker test/check_data_files.py groups fails to detect the following lines:

LONG
Formulas:
CPI = PM_RUN_CYC / PM_RUN_INST_CMPL
L1 DTLB 4K misses PM_DTLB_MISS_4K
--
descriptive text

It prints:

<LIKWID_SRC>/groups/power8/TLB_DATA.txt:26:1:
Expected {"-" ^ "--"}:
L1 DTLB 4K misses PM_DTLB_MISS_4K
^

While if the first formula contains no =, it prints:

<LIKWID_SRC>/groups/power8/TLB_DATA.txt:25:1:
expected '=' sign: