`ADMM` initializer supports setting `f=None`

[bwohlberg]

While not documented, the ADMM class __init__ supports setting f=None. This capability is used in several example scripts, e.g.

scico/examples/scripts/denoise_l1tv_admm.py

Lines 62 to 64 in 029a595

	solver = ADMM(
	f=None,
	g_list=[g_loss, g_tv],

This support should either be documented or removed. The latter is perhaps cleaner since the same effect can be achieved by setting f to a ZeroFunctional object.

[bwohlberg]

Agreement reached to remove support for f = None.