Fix error with large charges

[jaimergp]

The Gaussian parser fails if one file has large charge (say, -11). In those cases, the Gaussian output does not feature a space between the = and the <charge> value:

 Charge =-11 Multiplicity = 1

As a result one less field is obtained after the .split() method on the line, resulting in an IndexError when accessing the multiplicity value at line 338. This PR provides an alternative way of obtaining the multiplicity, splitting by = first, instead of blanks.