parse_frequencies doesn't parse imaginary frequencies correctly.

[kfir4444]

Describe the bug
When parsing frequencies from Molpro's output, if the output has imaginary frequencies (as in the example output), then the function returns them as non-negative.
example:

>>> from arc.parser import parse_frequencies
>>> parse_frequencies("output.txt", software="Molpro")
array([2127.  ,  456.65,  691.4 , 1237.97, 1582.75, 2494.32])

How to reproduce
download the output example, and run:

>>> from arc.parser import parse_frequencies
>>> parse_frequencies("output.txt", software="Molpro")

In the Downloads directory. The output should be:

array([2127.  ,  456.65,  691.4 , 1237.97, 1582.75, 2494.32])

While a correct output is:

array([-2127.  ,  456.65,  691.4 , 1237.97, 1582.75, 2494.32])

Additional context
Output example