Inconsistent ExternalInsulin type enum and isExternalInsulin attribute

[avouspierre]

With the #93 the model PumpHistoryEvent is modified to add external insulin Type and a isExternalInsulin boolean :

struct	type

However, when a external insulin is added with the interface, only the externalInsulin bool is defined to true (the type = bolus and no externalInsulin) :

I also found a function never used by any code where isExternalInsulin = true

The externalInsulinType is used by NS (and only by NS) to build a nightscoutTreatement object with a function to deduce the type : https://github.com/nightscout/Open-iAPS/blob/8ebd52a69612977cf705c5843d71250eb23bb009/FreeAPS/Sources/APS/Storage/PumpHistoryStorage.swift#L260

My proposal will be to remove the externalInsulin Type in Event Type and create a specific type enum for NS, avoid to have inconsistent between type and isExtrernalInsulin attribue.

[avouspierre]

tested with #110 PR