Introduce setCurrentLevel, getCurrentLevel and measureCurrent (and similar for voltage).

Addresses issue #20 (closed).

Changes the set/get functions the the following:

• IPowerSupply::setVoltageLevel: set voltage level (used to be setVoltage)
• IPowerSupply::getVoltageLevel: get currently set voltage level
• IPowerSupply::measureVoltage: measure the actual output voltage (used to be getVoltage)
• similarly for the current functions

In addition, the code is refactored such that:

• All voltage functions are together and all current functions are together. Doxygen groups are also used to group these functions in documentation.
• The code looks as similar between different implementations as possible.

The powersupply command now has the following commands:

• set-voltage V [I]: Set voltage V [V] with maximum current I [A] (same as before)
• get-voltage: Get set voltage level [V] (new, reuse a previous command name)
• meas-voltage: Get reading of voltage [V] (new, used to be get-voltage)