Add an abstract class which automates tabulation of parametric, spherically-symmetric, collisionless mass distributions

[abensonca]

• Works for any parametric, spherically-symmetric, collisionless mass distribution
• Class must specify methods:
  • density() - gives $\rho(r)$ (optionally also densityGradientRadial() - easy to derive);
  • parameters() - returns $\hat{\rho}$, $\hat{r}$, $\mathbf{p}$ (density normalization, radius normalization, other parameters);
  • factoryTabulation() - returns an instance of the class with $\hat{\rho}=1$, $\hat{r}=1$, and whatever $\mathbf{p}$ is provided to it - used to perform numerical integrations to build the tables;
  • suffix() - a suffix for use in tabulation file names (typically an MD5 hash generated automatically from the source code)
• Handles $M(r)$, $\phi(r)$, ℘₁,₂,₃ $(r)$, $\sigma(r)$, $F(k)$, $E(r)$, $r_\mathrm{ff}(t)$, $\mathrm{d}r_\mathrm{ff}/\mathrm{d}t(t)$;
• Can override any of these with analytic solutions if available;
• Results automatically tabulated (in parallel) over required range, stored/restored to/from file.

[github-actions]

⚠️ Possible misspelled word 'retabulated' in file 'source/mass_distributions.spherical.tabulated.F90'
⚠️ Possible misspelled word 'tabulatd' in file 'source/mass_distributions.spherical.tabulated.F90'
⚠️ Possible misspelled word 'pre' in file 'source/mass_distributions.F90'
⚠️ Possible misspelled word 'pre' in file 'source/mass_distributions.spherical.F90'
⚠️ Possible misspelled word 'pre' in file 'source/mass_distributions.spherical.F90'

[github-actions]

⚠️ Possible misspelled word 'retabulated' in file 'source/mass_distributions.spherical.tabulated.F90'
⚠️ Possible misspelled word 'pre' in file 'source/mass_distributions.F90'
⚠️ Possible misspelled word 'pre' in file 'source/mass_distributions.spherical.F90'
⚠️ Possible misspelled word 'pre' in file 'source/mass_distributions.spherical.F90'

[github-actions]

⚠️ Possible misspelled word 'retabulated' in file 'source/mass_distributions.spherical.tabulated.F90'
⚠️ Possible misspelled word 'pre' in file 'source/mass_distributions.F90'
⚠️ Possible misspelled word 'pre' in file 'source/mass_distributions.spherical.F90'
⚠️ Possible misspelled word 'pre' in file 'source/mass_distributions.spherical.F90'