Abstract repeated `get_uncertainty` functions

[jthorton]

Is your feature request related to a problem? Please describe.

In #2000 I noticed a lot of results classes use the std of the estimates to give the uncertainty which leads to a lot of repeated code it would be great to abstract this out to a single place if possible

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Additional context

[IAlibay]

Hopefully once we have #1717 we can probably use the same abstract base class for results - at that point the only difference between ABFE/SepTop/HybridTop will be restraints handling.

[jthorton]

The CLI also has repeated functions for the uncertainty which got missed in #2000
ABFE -

openfe/src/openfecli/commands/gather_abfe.py

Line 151 in d9c0153

def _error_std(r):

SepTop -

openfe/src/openfecli/commands/gather_septop.py

Line 164 in d9c0153

def _error_std(r):

It would be best to reuse the protocol result estimates where possible rather than implement the functions again in the CLI.