Add dedicated interpolate function

[Sfonxu]

This PR is a first in series of PRs of API refactoring described in #510.

The aim is to replace repeated definitions of interpolate around the code to a single function in PyMPDATA.impl.

Current tasks:

• Replace interpolate as in shallow-water examples
• Make interpolate JIT-compilable
• Look for other places where such functions are defined
• Look into replacing interpolate in those places

[slayoo]

* [ ]  Look for other places where such functions are defined
* [ ]  Look into replacing interpolate in other places, such as `Shipway and Hill (2012)`

The interpolation happens only when advecting momentum. Shipway and Hill (2012) example features only vapour mass advection, so it is not relevant. Other locations where we can refactor the interpolation code are:

• PyMPDATA/examples/PyMPDATA_examples/burgers_equation/burgers_equation.py

  Line 40 in a921acc

  return 0.5 * ((vel[indices] - vel[indices - 1]) / 2 + vel[:-1]) * dt / dx

• PyMPDATA/examples/PyMPDATA_examples/Jaruga_et_al_2015/fig19.ipynb

  Line 347 in a921acc

  "def calc_gc_interpolate_in_space(ARRAYS,ADVECTOR_DATA):\n",

• PyMPDATA/examples/PyMPDATA_examples/Jarecka_et_al_2015/simulation.py

  Line 40 in a921acc

  def interpolate(psi, axis):

[codecov]

Codecov Report

❌ Patch coverage is 77.35849% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.74%. Comparing base (a921acc) to head (5b9f157).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
PyMPDATA/impl/formulae_divide.py	0.00%	21 Missing ⚠️
PyMPDATA/impl/indexers.py	88.88%	2 Missing ⚠️
PyMPDATA/solver.py	96.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #612      +/-   ##
==========================================
- Coverage   92.69%   90.74%   -1.96%     
==========================================
  Files          39       31       -8     
  Lines        1697     1566     -131     
==========================================
- Hits         1573     1421     -152     
- Misses        124      145      +21     

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[Sfonxu]

It seems that, according to numba docs, the argument axis in np.diff is not supported.

[slayoo]

It seems that, according to numba docs, the argument axis in np.diff is not supported.

IIUC, usage of np.diff is anyhow out of question here as it allocates new memory

[github-actions]

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