Clean-up code of one-letter identifiers

The whole code base is littered with identifiers (variables, struct types, and types) made of a single letter.

To make the library code more readable, and easier to maintain and expand, these identifiers must be replaced with more telling names.

The only place where single letter variables can be accepted is for a counter in a short loop.

Moreover, variables and field name vdc and vcd should be avoided as they are easily mixed up, and replaced with vcDirector and vdCenter or similarly telling names.