`sse_states_to_endemicity()` and `endemicity_to_sse_states()` are non-symmetrical

[joshwlambert]

I would expect sse_states_to_endemicity() and endemicity_to_sse_states() to reverse each others output. However, this is not the case.

endemicity_to_sse_states(endemicity_status = "nonendemic", sse_model = "geosse")

gives 0.

However

sse_states_to_endemicity(states = 0, sse_model = "geosse")

gives an error and says states should be 1, 2 or 3.

sse_states_to_endemicity(states = 3, sse_model = "geosse")

gives "0" (as a character) which is weird behaviour.

Perhaps all of this behaviour is intended. If so just let me know and we can document the functions to reflect that and I'll close the issue.

[TheoPannetier]

Sorry, I somehow missed this notification in January! Yes, both outputs are wrong,

endemicity_to_sse_states(endemicity_status = "nonendemic", sse_model = "geosse")

should return 0 (numeric) and

sse_states_to_endemicity(states = 3, sse_model = "geosse")

should throw an error, while

sse_states_to_endemicity(states = 0, sse_model = "geosse")

should return "nonendemic" (as of now it errors). Let me create some tests and fix the issue.

[TheoPannetier]

Fixed in #24