Make activation function configurable as argument

[marianovitasari20]

Previously, activation functions were hardcoded in the blocks.
Now they can be set via the configuration, allowing different models to use different activations without changing code.

[github-actions]

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
ice_station_zebra/models/common
activations.py					1-3
bottleneckblock.py					1-20
convblock.py					1-32
upconvblock.py					1-24
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[jemrobinson]

Could we change the type-signature of these modules to take str instead of the function itself? This makes it easier to serialise for checkpointing.

[jemrobinson]

LGTM

@marianovitasari20: I've replaced the get_activation function with a direct call to the dictionary, so we can pass arguments (like inplace=True) when we instantiate the classes, as was done before.