Return the measurement index of the start of a given board observation

SYNOPSIS

    m = mrcal.cameramodel('xxx.cameramodel')

    optimization_inputs = m.optimization_inputs()

    x = mrcal.optimizer_callback(**optimization_inputs)[1]

    Nmeas   = mrcal.num_measurements_boards (   **optimization_inputs)
    i_meas0 = mrcal.measurement_index_boards(0, **optimization_inputs)

    x_boards_all = x[i_meas0:i_meas0+Nmeas]

The optimization algorithm tries to minimize the norm of a "measurements" vector
x. The optimizer doesn't know or care about the meaning of each element of this
vector, but for later analysis, it is useful to know what's what. The
mrcal.measurement_index_...() functions report where particular items end up in
the vector of measurements.

THIS function reports the index in the measurement vector where a particular
board observation begins. When solving calibration problems, most if not all of
the measurements will come from these observations. These are stored
contiguously.

In order to determine the layout, we need quite a bit of context. If we have
the full set of inputs to the optimization function, we can pass in those (as
shown in the example above). Or we can pass the individual arguments that are
needed (see ARGUMENTS section for the full list). If the optimization inputs and
explicitly-given arguments conflict about the size of some array, the explicit
arguments take precedence. If any array size is not specified, it is assumed to
be 0. Thus most arguments are optional.

ARGUMENTS

- i_observation_board: an integer indicating which board observation we're
  querying

- **kwargs: if the optimization inputs are available, they can be passed-in as
  kwargs. These inputs contain everything this function needs to operate. If we
  don't have these, then the rest of the variables will need to be given

- lensmodel: string specifying the lensmodel we're using (this is always
  'LENSMODEL_...'). The full list of valid models is returned by
  mrcal.supported_lensmodels(). This is required if we're not passing in the
  optimization inputs

- do_optimize_intrinsics_core
  do_optimize_intrinsics_distortions
  do_optimize_extrinsics
  do_optimize_frames
  do_optimize_calobject_warp
  do_apply_regularization

  optional booleans; default to True. These specify what we're optimizing. See
  the documentation for mrcal.optimize() for details

- Ncameras_intrinsics
  Ncameras_extrinsics
  Nframes
  Npoints
  Npoints_fixed
  Nobservations_board
  Nobservations_point
  calibration_object_width_n
  calibration_object_height_n

  optional integers; default to 0. These specify the sizes of various arrays in
  the optimization. See the documentation for mrcal.optimize() for details

RETURNED VALUE

The integer reporting the variable index in the measurements vector where the
measurements for this particular board observation start
