store default data per activity

[marcopeg]

each activity should be identified by an ID and should expose a json definition for it's default details.

{
  duration: xxx
  unit: repetition | seconds
  points: xxx
  ...
}

then in each session definition it is possible to override those values

[marcopeg]

there should be 3 levels of activity definition:

global activity

they are hard coded into the application as json files.
identified by a string ID es. "pushups"

session activity

each training-session definition file can define it's own set of activities which share the same structure of the global activity.

if a session activity declare the very same ID it will extend the global activity

step activity

the training-session file exposes the schedule which is a list of activities to be executed.

each item can be a brand new activity or an extension of a session activity with a fallback to the global activity