Configf: add multiple index (unsigned int) read method.

[stuart-knock]

A limitation of Configf is that it has only one member function, numbers(), for reading multiple numbers(vector or array) from a configuration file. This member returns a vector of doubles, which causes problems with type specification when what we are reading are indices (eg, node index to apply stimulus to).

This should be corrected. Probably best to start by renaming numbers() to something more specific like read_doubles() and adding a new member function for reading and returning unsigned int like size_type.

Only two should be required as indices(size_type) and doubles are the only type of content we specify in vectors...

Once done, the remaining instances of indices being declared as double instead of size_type can be corrected.