Launch jobs with variable nice value

Description

Based on the discussion in https://git.xfel.eu/calibration/planning/-/issues/110, this MR introduces a flexible --nice value passed to sbatch depending on the number of jobs running in the selected partition for this instrument. Originally only meant for active proposals, the code is cleaner when just using the partition value determined before and is currently changed by ITDM's script anyway shortly after submission. I will clarify this point before merging.

How Has This Been Tested?

Tests! Unit tests!

Types of changes

• New feature (non-breaking change which adds functionality)

Reviewers

@roscar @kluyvert @ahmedk

tests/test_webservice.py

@@ -191,9 +191,9 @@ async def test_get_slurm_partition(proposal_number,
 async def test_get_slurm_nice(fake_process, cycle, num_jobs, expected_result):
     fake_process.register(
         ['squeue', '-h', '-o', '%.20j', '-p', 'exfel', '--me'],
-        stdout=b'\n'.join(
-            [f'correct_SPB_{i}'.encode('ascii') for i in range(num_jobs)] +
-            [f'correct_FXE_{i}'.encode('ascii') for i in range(num_jobs*2)]),
+        stdout='\n'.join(
+            [f'correct_SPB_{i}' for i in range(num_jobs)] +
+            [f'correct_FXE_{i}' for i in range(num_jobs*2)]).encode('ascii'),
         returncode=0)
 
     ret = await get_slurm_nice(