Report a branch whose reference parameters cannot be computed

[DavidSagan]

Every element takes its reference from the one before it, so the whole branch
rests on the first element's: it is either written on that element (a
BeginningEle's ReferenceP) or propagated in from the Fork that created the
branch. When neither happens there is no species and no energy to propagate, and
nothing later in the branch can supply them -- a ReferenceP further down is read
as that element's own upstream values, not as a starting point for the ones
before it. Until now the bookkeeper wrote a ReferenceP holding only time_ref: 0
and said nothing.

This is what lattice_files/fork.pals.yaml hits: b_fork sets
propagate_reference: false and b_line opens on m, a Marker with no
ReferenceP of its own, so the branch has no reference at all. It is now the one
problem that file reports.

Species and energy are reported separately, because either alone leaves the
reference unusable: the species supplies the mass that converts between energy
and momentum, and without one of those two there is nothing to convert.

The check runs on the first element after it has been bookkept, so it sees the
reference the element ended up with, fork seed and all, and the branch name is
in scope to name it by -- an element name alone would be ambiguous, since the
same definition can open several branches.

Five test fixtures declared no reference at all. Four are given one (they are
testing forks and expressions, not references); the fifth is the deliberately
broken lattice, where the new message joins the ones it already collects.

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