134 Slice `SteadyStateResult3D` to `SteadyStateResult2D`

Introduction

https://github.com/agencyenterprise/neurotechdevkit/issues/134 This PR adds the get_steady_state_result_2d method to the SteadyStateResult3D class to be able to convert the SteadyStateResult3D output of a 3-D simulation to a SteadyStateResult2D that just encapsulates the slice of interest.

Changes

The get_steady_state_result_2d method with optional arguments of slice_axis and slice_position (the slice_axis and slice_position property of the 3D result is used when none is given) is added to the SteadyStateResult3D class in _results.py. The method _validate_slice_args is also added to SteadyStateResult3D to check the slice being made. The SteadyStateResult2D object returned by the get_steady_state_result_2d method has the target and sources removed (should be changed to the correctly sliced target and sources in https://github.com/agencyenterprise/neurotechdevkit/issues/148); warnings for missing target/sources are added to the rendering and metrics of SteadyStateResult2D. Moved definition of SliceAxis enum from _base.py to _utils.py, so the get_steady_state_result_2d method can be typed.

Behavior

The new behavior is the generation of a 2D steady-state result object from a 3D steady-state result object by the get_steady_state_result_2d method of the SteadyStateResult3D class. This can be tested by making a 3D steady-state result and calling the method to generate a 2D slice of that result.

Files Changed	Coverage
src/neurotechdevkit/results/_results.py	`78.72%`
src/neurotechdevkit/scenarios/__init__.py	`100.00%`
src/neurotechdevkit/scenarios/_base.py	`100.00%`
src/neurotechdevkit/scenarios/_utils.py	`100.00%`
.../neurotechdevkit/scenarios/built_in/_scenario_0.py	`100.00%`
.../neurotechdevkit/scenarios/built_in/_scenario_1.py	`100.00%`
.../neurotechdevkit/scenarios/built_in/_scenario_2.py	`100.00%`

agencyenterprise / neurotechdevkit