Allow different recurrence threshold for 2nd matrix in Joint recurrence matrix

[Datseris]

see also JuliaDynamics/DynamicalSystems.jl#210

Would be nice to allow a 4th argument εy for the recurrences of y.

Only question is: what would be the recurrence type stored and displayed by the joint matrix? At the moment we store ε and we use it when we display the matrix. I guess we could make a new subtype called only for display purposes.

[Datseris]

Probably best to add a second field to the joint matrix type and display both recurrences types. Then again, this isn't scalable to joint recurrence of three types.

So, first, we have to allow constructor JointRecurrenceMatrix(VarArg{RecurrenceMatrix}) so that you can do JRM(Rx, Ry, Rz, ...). Then, the field of hte recurrence type becomes a tuple of the recurrence types of each of the input matrices. For display, I think we should only display the recurrence type if it is the same for all matrices.