Add Initialisation, Base.resize!, Base.append! and concatenation to interface

[Luapulu]

I had some ideas for possible additions:

It might be cool for something like arr = SomeDiskArray{Float64}(file_handle, undef, 100, 10) to be part of the interface. This would make it possible to treat a DiskArray almost exactly like an Array. Functions like map would "just" work. May be even go so far as to allow arr = SomeDiskArray{Float64}(undef, 100, 10) where the file object is automatically created in the current working dir with some temp name.

If a DiskArray implements the resize! method, we can have an automatic append! be defined for any DiskArray.

For concatenation, we can define a cat!(out, A..., dims=dims) function. If out can be resized, it can be any size, if it cannot, the size of out must be the sum of the sizes of all A. May be use a Trait to specify whether or not a DiskArray can be resized. If we have initialisation defined, we can even have cat work like it does for normal arrays. It'll just initialise a new array and write into it.