decouple setting global neuron attributes from sim run [vcs: #minor]

[anilbey]

Separates global neuron simulator attributes such as temperature from the simulation class. Calling simulation.run without specifying temperature no longer overrides neuron.h.temperature with this change.

addresses #153.

The usage is the following:

NeuronGlobals.get_instance().temperature = 34.0
sim.run(duration, cvode=False, dt=dt)

• Removes unreachable use_random123_stochkv=False setting from simulation.
• Sets the spike_threshold, check_for_spiking parameters at the parent process (lest the defaults become unmodifiable).
• Remove direct accesses to h.celsius or h.v_init from the code

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.85%. Comparing base (8bfa9b5) to head (c2f7536).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #154 +/- ## ========================================== - Coverage 90.86% 90.85% -0.02% ========================================== Files 89 89 Lines 5320 5292 -28 ========================================== - Hits 4834 4808 -26 + Misses 486 484 -2 ```

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[anilbey]

Just added 3 more lines for v_init in a test. Merging once CI succeeds.