Simplify memory_gpu field in methods database entry structure

[yousefmoazzam]

Current structure

YAML

memory_gpu: 
   - datasets: [tomo] 
   - multipliers: [2.1] 
   - methods: [direct] 

where the value of the memory_gpu field parses into the following python:

[
  {"datasets": ["tomo"]},
  {"multipliers": [2.1]},
  {"methods": ["direct"]},
]

Goal for simplified structure

YAML

memory_gpu: 
  datasets: "tomo"
  multipliers: 2.1
  methods: "direct"

where the value of the memory_gpu field parses into the following python:

{
  "datasets": "tomo",
  "multipliers": 2.1,
  "methods": "direct",
}

• Remove unnecessary single-element lists in methods database entries #356
• Replace unnecessary list of dicts in methods database entries #357

[team-gpu]

Also note that this loop will have to go too. Originally there was the option to have darks and flats fields there too.