Allow initial(.) on the rhs

[richfitz]

Should be possible to write things like:

deriv(x) <- exp(-(x - initial(x)^2)

given we always store the conditions. This will save having to route through a separate variable:

ix <- ...
initial(x) <- ix
deriv(x) <- exp(-(x - ix)^2)

[richfitz]

Also,

initial(t)

should work as we keep that too

[richfitz]

xref #5