Add intercept-only DyNAM-rate and REM model

[auzaheta]

Describe the bug

An only-intercept rate model produces an error.

To Reproduce

library(goldfish)
data("Social_Evolution")
callNetwork <- defineNetwork(nodes = actors, directed = TRUE)
callNetwork <- linkEvents(x = callNetwork, changeEvent = calls,
                          nodes = actors)
callsDependent <- defineDependentEvents(events = calls, nodes = actors,
                                        defaultNetwork = callNetwork)
# A individual activity rates model
mod02 <- estimate(callsDependent ~ 1,
                  model = "DyNAM", subModel = "rate")

> Parsing formula.
>   Error in strsplit(.split, split = "\\s*,\\s*") : non-character argument > 

Desktop (please complete the following information):

• OS: Windows 10 Education
• R Version 4.0.3
• Goldfish Version 1.5.0