Add tcl get methods for power information

[maliberty]

In The-OpenROAD-Project/OpenROAD-flow-scripts#2163 we need to be able to get the information from report_power in a TCL so that we can perform some checks. I see no commands in sta to do this.

[akashlevy]

sta::design_power [sta::parse_corner {}] should do the trick. I've used this in some of my own scripts with success

[jjcherry56]

Akash is correct. These functions have been there all along. Use sta::cmd_corner to get the default command corner or sta::find_corner for a specific corner rather than the arg parsing functions.