BUG: Update references to old function name

Timeseries plotting functions used the old name
_timeseries_mean_and_std, which had been changed to
_timeseries_mean_error. Now these references are updated.
DEMO: Import all demo modules automatically and add a function for running all demos
ENH: Implement efficient messaging to parents in SumMultiply

The message to parents is implemented such that it computes sums over
axes without computing the fully broadcasted message explicitly.
TST: Add tests for the message to parents in SumMultiply
API: Remove MatrixDot and references to it

MatrixDot was a class for computing the product of a Gaussian matrix and
vector. The more general SumMultiply has now replaced this class.
DEMO: Update demo for linear state-space model.
DEMO: Update PCA demo
DEMO: Update demo for categorical distribution