Updating CRS specifications in existing tutorials

[florisvdh]

This affects existing tutorials where proj4strings were used in defining coordinate reference systems (see #164). proj4strings are to be avoided nowadays.

In these commits it has been tried to:

• update CRS specifications in line with the advice of the spatial_crs_coding tutorial (Tutorial on current (more futureproof) CRS coding in R #196)
• add a little explanation on this where needed
• update existing explanations and documentation on CRS in these tutorials
• add a little word about proj4string warnings where they pop up (once warnings will occur no longer, some of the Rmd files are best rerun)

In some cases, YAML header, typos and linguistics were improved.

The impacted tutorials are:

• r_inla/spatial (@ThierryO)
• spatial_create_leaflet (@JeroenSpeybroeck)
• spatial_point_in_polygon (@stijnvanhoey , @hansvancalster)
• spatial_standards_raster (me)
• spatial_transform_crs (@ThierryO)

Fixes #164 .

[florisvdh]

I'd also like to draw attention to the following note that was added at the beginning of sp-oriented tutorials:

General note: migration to the more actively developed sf package is currently advised by the sp maintainer.
The sp package, used in this tutorial, is still maintained in order to support the newest versions of the GDAL and PROJ backends.

To the involved tutorial authors: I believe this is reason enough to plan augmenting your tutorial with an sf counterpart (suggestion: part 1 'with sf', part 2 'with sp').

• To remind yourself, please add a task yourself in this tutorials source repo (make separate issue as a reminder, add it to one of the predefined milestones, and click 'assign yourself').
• To actually do it, start a new branch to make the new commits and make a pull request (see tutorial on this).

[hansvancalster]

Thanks for taking the time to fix this. Looks good. I only checked the spatial point in polygon tutorial. I will open an issue to update the tutorial to use sf.