Fix several maxwell issues

Description

Fix memory leaks and access to uninitialized variables. Also some style fixes.

News snippet

Fix several Maxwell issues

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.

changed milestone to %11.0

added Bug + 1 deleted label

@heiko.appel @fbonafe I fixed the problems flagged by valgrind in !896 (merged) Since these are all Maxwell related, I thought it would be better to fix this in a separate merge request. Please have a look to see if I my fixes are okay.

Codecov Report

Merging #926 into develop will increase coverage by 0.00%. The diff coverage is 82.35%.

@@           Coverage Diff            @@
##           develop     #926   +/-   ##
========================================
  Coverage    71.06%   71.07%           
========================================
  Files          507      507           
  Lines        98778    98807   +29     
========================================
+ Hits         70194    70223   +29     
  Misses       28584    28584           

Impacted Files	Coverage Δ
src/hamiltonian/hamiltonian_mxll.F90	19.14% <ø> (-0.20%)
src/hamiltonian/maxwell_boundary_op.F90	49.15% <ø> (+1.56%)
src/td/propagator_mxll.F90	20.61% <40.00%> (+0.01%)
src/hamiltonian/energy_mxll.F90	97.29% <100.00%> (ø)

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approved this merge request

resolved all threads

mentioned in commit e37d40ad

merged