Improved measurements timings

[maij]

Fixes a bug that prevented certain timings from being recorded correctly.

Measurements now record the time between each measured value (stored as pre_msmt_time) which can be useful to pinpoint wasted time, especially with significant setup or analysis between measurements.

N.B. the time to save traces is handled by the layout, which is a layer above the qcodes measurement context, so we cannot easily include that as a separate value. The time taken to acquire a parameter (i.e. to perform layout.acquisition + analysis) includes the time to save traces.

Example code and timings:

dummy = Parameter('dummy', get_cmd=lambda: 1)
dummy2 = Parameter(get_cmd=lambda:2)

with Measurement('test') as msmt:
    for _ in Sweep(range(10)):
        msmt.measure(dummy)
        msmt.measure(dummy)
        sleep(0.333)
        msmt.measure(123, 'test_other_name')
        sleep(np.random.rand())
        msmt.measure(dummy2)
        msmt.measure({'hello': 4123, 'world':13}, 'dict')
        sleep(0.21)

As expected, the measurements with sleep commands before them have larger pre_msmt_times than the others.
This seems to work for all current measurables.

{ 'dataset': { 'finalize': '30.9+-0 ms',
               'save_metadata': '12.3+-18.7 ms',
               'store': '191+-303 us'},
  'measurement': { (0, 0): { 'dummy': '6.46+-18.6 ms',
                             'pre_msmt_time': '191+-59.9 ms',
                             'total': '197+-41.4 ms'},
                   (0, 1): { 'dummy': '620+-1.28e+03 us',
                             'pre_msmt_time': '11.6+-7.25 us',
                             'total': '706+-1.26e+03 us'},
                   (0, 2): { 'pre_msmt_time': '333+-0.112 ms',
                             'test_other_name': '896+-1.77e+03 us',
                             'total': '335+-1.69 ms'},
                   (0, 3): { 'None': '1.44+-2.33 ms',
                             'pre_msmt_time': '542+-325 ms',
                             'total': '543+-324 ms'},
                   (0, 4): { 'dict': '2.09+-4.26 ms',
                             'pre_msmt_time': '12.9+-3.42 us',
                             'total': '55.2+-11.6 us'},
                   (0, 4, 0): { 'hello': '915+-2.19e+03 us',
                                'pre_msmt_time': '420+-1.35e+03 us',
                                'total': '1.13+-2.2 ms'},
                   (0, 4, 1): { 'pre_msmt_time': '42.1+-12.3 us',
                                'total': '941+-2.06e+03 us',
                                'world': '836+-2.05e+03 us'}}}