Including a add_index to allow custom index integration into workflow

[MartinuzziFrancesco]

currently the compute and compute_index function only work when the index is in the indices list. This makes computing custom indices kinda wonky, so the addition of a add_index function would help. This function should update the indices Dict as well as the indices_funcDict.

[MartinuzziFrancesco]

Actually we don't need this, we can just dispatch this over a function compute_index(index::AbstractSpectralIndex, params=nothing, online::Bool=false) that works as internal call to compute_index(index::String, params=nothing, online::Bool=false)

[MartinuzziFrancesco]

this was addressed in #58