performance: generics support

[Mathis Derenne]

1. Define Type generic : numeric, text, bool, undefined (being nil value of string).
2. Load lines of jsonlines till colType is known for every columns, if a colType is undefined : log.warn() + load value as []undefined
3. Load lines and append to a list of said type.
4. Use []T throughout construction of MainMetric()
5. Use []numericT, []textT, []boolT as type signature input of SetTypeMetric()

Scope of impact :

• performance gain : []interface{} is slow and type assertion v.(type) is used in a lot of functions. Generics will remove this need.
• typed data : less error