rename parameter types to more intuitive names

[annecori]

e.g. change "Human delay - serial interval" to "serial interval" (I think the default can be human)

[adamkucharski]

We also discussed having an additional R friendly naming column (e.g. human_delay_serial_interval)

[kellymccain28]

Parameter types are also not the same across pathogens, e.g.
in Ebola parameter dataset, recorded as "Human delay - Symptom Onset/Fever to Death" whereas in Lassa parameter dataset, recorded as "Human delay - symptom onset>death"

[sangeetabhatia03]

Parameter types being different is an issue that should be resolved urgently I think, given we have released Ebola and Lassa datasets