Validate PET data objects' attributes at instantiation: ensures that the
attributes are present and match the expected dimensionalities.

Refactor the PET attributes so that only the required (`frame_time` and
`uptake`) and optional (`frame_duration`) parameters are accepted by the
constructor. The `midframe` and the `total_duration` attributes can be
computed from the required parameters, so exclude it from `__init__`.

Although `uptake` can also be computed from the PET frame data, the
rationale behind requiring it is similar to the one for the DWI class
`bzero`: users will be able compute the `uptake` using their preferred
strategy and provide it to the constructor. For the `from_nii` function,
if a callable is provided, it will be used to compute the value;
otherwise a default strategy is used to compute it.

Refactor the `from_nii` function so that the required parameters are
present when instantiating the PET instance. Increase consistency with
the `dmri` data module `from_nii` counterpart function.

Refactor the PET data creation fixture in `conftest.py` to accept the
required/optional arguments and to return the necessary data.

Refactor the tests accordingly and increase consistency with the `dmri`
data module testing helper functions. Reduces cognitive load and
maintenance burden.

Add additional object instantiation equality checks: check that objects
intantiated through reading NIfTI files equal objects instantiated
directly.