Unit TPS constructor needed

[David Sagan]

Instead of constructing unit TPS variables one at a time, it would be convenient to be able to create an array at once.  Something like:
x = unit_maps(descriptor)
So that x[1], x[2], ..., x[N] are unit maps for variables 1, 2, ..., N. With this, a TPS can be constructed like:
y = 2 + 4.3*x[1]^2 + x[3]

[mattsignorelli]

This has been implemented in vars, params, complexvars, and complexparams.