Enums should have a defined type

[mgeplf]

#243 Fixes a type conversion, but since C++11, we can specify the type, to make this more clear.

[wizmer]

Pinging @jacquemi-bbp because this can be part of #242.
This PR will transform the SectionType enum into a class enum so we can take this opportunity to also give them a type.

The reason is that glia will have a new enum to store their own section type (instead of using a SectionType enum which currently holds values for both the neurons and the glia as done now).
In this context, if glia and neuron want to use the same label (for example, if both need a type call "undefined") this is not possible without using class enums.

[jacquemi-bbp]

+1
spine will also have a new enum to store their own section type