Add convolution operator ApproxFun.jl#940

[Jilhg]

This implements a convolution operator on (real) periodic segments (see JuliaApproximation/ApproxFun.jl#940). It works for Laurent and Fourier spaces. Also added some testcases.

As I am relatively new to this, please feel free to give any feedback. If you prefer some different style I am happy to change.
In particular:

• Is it better to override getindex or defaultgetindex?
• Is the description of the Convolution.jl file sufficient? Also, how can I add it to the docs?
• Should I provide an example (and if so, where)?

[jishnub]

Sorry for the delayed message. I think it's better to extend getindex directly instead of defaultgetindex.

[Jilhg]

Ok, thank you, I updated this