Indexed container parameters

[mormj]

Need some way to have setters and getters for block parameters that allow element access. Example: say, it is a uhd_block, but I want to have a parameter that represents gain. This could be a vector of gains where each element sets a particular channel. But then when I do set_gain(x), the current mechanism expects x to be a vector and set all the gains in one shot.

Instead I should be able to do set_gain(0,x) where x is a float