add a section on point sampling and population estimates

[aspina7]

in order for the manual to link to other material and not cover too much technical stuff on GIS we should add to the epirhandbook with a section on spatial point sampling, and population estimates.
This will already be part of the sampling chapter - but adding code for population estimates is important
https://epiet.github.io/RapidAssessmentSurveys/gis_sampling.html

[aspina7]

cross-linking to this #102