increase version number
add getters and setters for subgroups
add basic support for simulating subgroups
improve unittest
document missing arguments
adapt parse_trees to support subgroups
update documentation
implement subgroups for ms and msms
improve unittests
update test and documentation
made sizes of subgroup random
implement gamma rate variation for selection
add variable rec and mut rates
update NEWS & man
Merge branch 'gamma_rates' into develop

Conflicts:
	R/demographic_model.R
	tests/testthat/test-u-demographic-model-class.R