Protection of existing units in the existing environment

[thisisapple]

Problem: The user accidentally defines an existing unit. It can lead to mistakes.

Feature needed User should not be allowed to overwrite the existing units in the selected environment.

Example codes

import forallpeople as si
si.environment('structural',top_level=True)
psf = 144 * psi

%%render
sigma = 513*psf
A = 5 * ft**2
F =  sigma * A
F = F.to('kip')