The data field from SampleBuf and SpectrumBuf now accept `Abstr…

[pjabardo]

…actArray`

Used the approach used by AxisArrays where the a new parametric type is used to specify the array data type. The constructor is used to garantee that an AbstractArray is actually used.

What does this implement/fix?

For now, SampleBufP and SpectrumBuf use an Array object to store data. But using an AbstractArray could be interesting, mostly for using views of arrays.

There is an issue #60 that deals with it but the approach
uses abstract type for the data field which has several issues.

The approach here, uses the same approach as AxisArrays pacakge:

mutable struct SampleBuf{T, N, D} <: AbstractSampleBuf{T, N}
    data::D
    samplerate::Float64
    SampleBuf{T,N,D}(data::AbstractArray{T,N}, sr::Real) where {T,N,D} = new{T,N,D}(data,sr)
end

Now, not only can SubArrays be used but other arrays types such as StaticArrays or FixedSizeArrays can be used.

Reference issue

Example: Fixes #60.

Additional information

Any additional information you think is important.