measure_thermal_behavior: temp_data restructure, tweaks

[alchemyEngine]

Reformat temp_data data structure as e.g:

    "temp_data": {
        "2021/08/02-18:09:24": {
            "ambient_t": 25.045069874183582,
            "bed_target": 105.0,
            "bed_temp": 70.55504880410373,
            "chamber_temp": 28.64916624748446,
            "frame_temp": 25.01276217159301,
            "he_target": 100.0,
            "he_temp": 100.10888741760135,
            "mcu_z": -5587,
            "sample_index": 0
        },
        "2021/08/02-18:09:28": {
            "ambient_t": 25.01276217159301,
            "bed_target": 105.0,
            "bed_temp": 70.96215755085439,
            "chamber_temp": 28.462018360494767,
            "frame_temp": 24.899409750362793,
            "he_target": 100.0,
            "he_temp": 99.61074527343162,
            "mcu_z": -5586,
            "sample_index": 0
        }
   }

Move start_time to after bed is heated, remove extra start_time from before cold measurements.