MAINT: Explicitly raise when the line with the number of orbitals doesn't have any actual orbitals

[BvB93]

There seems to be a CP2K bug (?) as of 8.2 where the number of occupied orbitals isn't always printed.
Explicitly raise an exception when this occurs.

Examples

 Number of electrons:                                                          2
 Number of occupied orbitals:                                                    <--- where are they!???

 TOTAL NUMBERS AND MAXIMUM NUMBERS

[codecov]

Codecov Report

Merging #295 (4326fd3) into master (23589d8) will decrease coverage by 0.04%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #295      +/-   ##
==========================================
- Coverage   82.58%   82.53%   -0.05%     
==========================================
  Files          59       59              
  Lines        2492     2497       +5     
  Branches      417      418       +1     
==========================================
+ Hits         2058     2061       +3     
- Misses        334      335       +1     
- Partials      100      101       +1     

Impacted Files	Coverage Δ
src/qmflows/parsers/cp2k.py	72.40% <66.66%> (-0.21%)	⬇️

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