Document difference between ZeroMatrix & NewZeroMatrix; ZeroVector & NewZeroVector; IdentityMatrix & NewIdentityMatrix

[fingolfin]

... or else clarify how they relate to each other (e.g. which is implemented in terms of which, and for which ones does one need to provide methods in order for a new MatrixObj type to work.

[fingolfin]

OK, after sleeping on this and looking again, I now see why the two exist: NewZero* are constructors (taking a filter as first argument), the others are regular operations. OK, got it. So we probably want to keep both, but should point out the difference explicitly, and also say which is implemented in terms of which (if at all).

[fingolfin]

In PR #4366 I am removing most ZeroMatrix/IdentityMatrix methods, keeping just one for each set of argument types; they all dispatch to NewZeroMatrix / NewIdentityMatrix. However, I left ZeroMatrix/IdentityMatrix as operations, even though I was tempted to turn them into a single global function each; the reason is that the cvec package installs methods for the variants getting an example matrix, as an optimization: each of its matrices stores a "CVecClass" object, and it can then reuse these. Anyway, whatever the reasons, that means these need to stay operations for now, and if want to change it, we need to coordinate it with the cvec package.