A convex subset of a parameter space: a family's own \(\Theta\),
or one (possibly overlapping) piece \(\Theta_{0i}\) of a null
\(\Theta_0 = \bigcup_i \Theta_{0i}\). It
provides dimension, membership, projection and a chart() of constrained
coordinates for optimisers.
Examples
# `convex_region` is abstract; polytope_region(), simplex_region(),
# halfspace_region(), point_region() and real_region() subclass it:
s <- simplex_region(vertices = diag(3))
S7::S7_inherits(s, convex_region)
#> [1] TRUE
space_dim(s)
#> [1] 3