Refactor HMC initialisation code

[penelopeysm]

I'm really wary of #2566 becoming very big, so I'm trying to break off smaller bits.

This is a really basic bit of refactoring that I think just makes things easier to read.

Anyway, I've tested before and after with

using Turing, Random
@model f() = x ~ Normal()
sample(Xoshiro(468), f(), NUTS(), 1000)

and the exact numerical result is unchanged, so I'm pretty sure that there isn't actually any code change — hence the lack of a version bump.

┌ Info: Found initial step size
└   ϵ = 3.2
Sampling 100%|██████████████████████████████████████████████████████████████████| Time: 0:00:03
Chains MCMC chain (1000×13×1 Array{Float64, 3}):

Iterations        = 501:1:1500
Number of chains  = 1
Samples per chain = 1000
Wall duration     = 4.59 seconds
Compute duration  = 4.59 seconds
parameters        = x
internals         = lp, n_steps, is_accept, acceptance_rate, log_density, hamiltonian_energy, hamiltonian_energy_error, max_hamiltonian_energy_error, tree_depth, numerical_error, step_size, nom_step_size

Summary Statistics
  parameters      mean       std      mcse   ess_bulk   ess_tail      rhat   ess_per_sec
      Symbol   Float64   Float64   Float64    Float64    Float64   Float64       Float64

           x   -0.0338    0.9883    0.0494   396.8876   699.6500    1.0029       86.5433

Quantiles
  parameters      2.5%     25.0%     50.0%     75.0%     97.5%
      Symbol   Float64   Float64   Float64   Float64   Float64

           x   -1.9956   -0.6683   -0.0578    0.6140    1.9267

[github-actions]

Turing.jl documentation for PR #2567 is available at:
https://TuringLang.github.io/Turing.jl/previews/PR2567/

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.86%. Comparing base (411a341) to head (39ad15e).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/mcmc/hmc.jl	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2567      +/-   ##
==========================================
- Coverage   85.92%   85.86%   -0.07%     
==========================================
  Files          21       21              
  Lines        1428     1429       +1     
==========================================
  Hits         1227     1227              
- Misses        201      202       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[coveralls]

Pull Request Test Coverage Report for Build 15307178307

Details

• 14 of 14 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 85.924%

---

Totals
Change from base Build 15285321138:	0.0%
Covered Lines:	1227
Relevant Lines:	1428

---

💛 - Coveralls

[mhauru]

LGTM