Add mean centered phi flag for HSGP classes

[wd60622]

Provide flag for which centers the basis phi = phi - phi.mean(axis=0).eval()

pymc-marketing/pymc_marketing/mmm/hsgp.py

Lines 751 to 753 in 3cccdf0

	phi, sqrt_psd = gp.prior_linearized(
	self.X[:, None] - self.X_mid,
	)

Would also be done for the each basis in Periodic class too?

pymc-marketing/pymc_marketing/mmm/hsgp.py

Lines 973 to 975 in 3cccdf0

	(phi_cos, phi_sin), psd = gp.prior_linearized(
	self.X[:, None] - self.X_mid,
	)

Maybe the wise @bwengals can provide some guidance on this. The questions that I have are:

• If doing this for the HSGP Periodic class, this would be done for both sin and cos bases, correct?
• Is this just an alternative to the drop_first flag? (removing intercept-like basis). Does it provide some additional benefit / stability to the model?