iss39 Implemeted edge triggered counter

[TM90]

This pull request closes #39

• Added edge_trigger function to rfgfunctions.tcl
• Implemented edge_trigger in RegisterFile Generator
• Implemented edge_trigger in rfs_to_rfg tool

A counter field which is edge triggered is described like this:

field counter_field {
    width 32
    software {
        rw
    }
    hardware {
        counter
        edge_trigger
    }
}