SetupTDCS

I would like to rename this module from
"SetupRHSforTDCSandTMS"  -> "SetupTDCS"
Can't do this on Windows yet--change to:
```
static const int special_label = -4321;
static const double electode_current_summation_bound;
```
in the header and then initialize the double in the .cc file:
```
const double SetupRHSforTDCSandTMSAlgorithm::electode_current_summation_bound = 1e-6;
```