Runtime dynamic access

[perrydv]

This PR provides support for obj[[varname]] where obj is an nClass object and varname is a string to access a variable in C++ by name at runtime. This returns an ETaccessorBase (ET is for "Eigen Tensor". "Base" is for "base class".) That object can only be used through an as statement, e.g. as(obj[[varname]], 'numericVector'). Even if obj[[varname]] is not a numericVector, it will be handled as one if possible (i.e. if at runtime it is conformable to a vector, such as a higher dimensional tensor with all but one extent being 1). This PR also extends as to handle true scalar types as the input or output type.

[perrydv]

Also this has passed tests under Actions, so I will plan to merge it soon.

[paciorek]

Do we have "char" or "string" as a type, e.g.,

function(varname = "string") {
obj[[varname]] ....
}

[perrydv]

Yes.

Adding features for vectors (or potentially tensors) of strings is somewhere down the road, but we do have a scalar string type.