in `meshTools` combine/consolidate the multiple smoothing / transferring scalars functions

[gattia]

As it stands there is currently:
Smoothing scalars:
- Gaussian : gaussian_smooth_surface_scalars
- Weighted average : get_smoothed_scalars
Transfer scalars:
- Gaussian : smooth_scalars_from_second_mesh_onto_base
- Weighted average : transfer_mesh_scalars_get_weighted_average_n_closest

These could likely be consolidated. Also, the naming conventions are all over the place - these should be similar/related in an easily interpretable way.