Dubious type casting for augmented arithmetic operators

We currently allow, and validate via testing in test_Data_BINARY_AND_UNARY_OPERATORS, some behaviour relating to input and output data types for augmented arithmetic assignment operators that is not allowed by NumPy, and we should consider whether this is suitable or not. I am inclined to say we should make appropriate changes to adopt the NumPy behaviour.

Specifics

Namely, when an augmented assignment is performed using inputs with data types which lead to a change in data type for the output, e.g. for a simplified scalar case something like a = 1; a += 1.0, we permit an in-place change of array dtype. As a minimal example, note how NumPy raises a type casting error for the equivalent operation below, whereas we go ahead and produce an output with a changed data type, the same type that the operation not in-place would produce:

>>> import cf
>>> import numpy as np
>>> 
>>> # Setup equivalent arrays
>>> i_np = np.array([1, 2, 3])
>>> i_cf = cf.Data(i_np)
>>> 
>>> # NumPy raises a type casting error:
>>> i_np + 1.0  # operation not in-place is fine
array([2., 3., 4.])
>>> i_np += 1.0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
numpy.core._exceptions.UFuncTypeError: Cannot cast ufunc 'add' output from dtype('float64') to dtype('int64') with casting rule 'same_kind'
>>> 
>>> # ... whereas cf performs the operation to give the same result data type
>>> # as the non in-place operation would:
>>> i_cf + 1.0
<CF Data(3): [2.0, 3.0, 4.0]>
>>> i_cf += 1.0
>>> i_cf
<CF Data(3): [2.0, 3.0, 4.0]>

and the equivalent behaviour occurs for the various __i<operator>__ operators.

Relevant cases in test suite

For reference, the tests in test_Data_BINARY_AND_UNARY_OPERATORS which were checking for this (dubious) behaviour, which remain as such from before the LAMA to Dask migration, are:

https://github.com/NCAS-CMS/cf-python/blob/003374315b37568f8594db0bbbab3b5d5c31e0b8/cf/test/test_Data.py#L2062-L2154

Start by reading and running the referenced cases in cf/test/test_Data.py, especially test_Data_BINARY_AND_UNARY_OPERATORS around lines 2062–2154, and reproduce the NumPy comparisons in the issue. Done means the desired augmented-assignment casting policy is decided and the implementation and tests consistently reflect it.