New dependency: python3-progressbar

Add label method to Resistor and Collection; fix combinations returning single resistors when not requested

Label method provides options for printing tolerance and constituents.
Fixed Set.combinations() returning single resistors even when num_series and num_parallel were > 1.

The code previously did not consider oppositely-oriented resistor pairs, i.e.
R2/R1 as well as R1/R2. The use of `itertools.permutations` fixes this.

Instead of using sets, which are unwieldy for large numbers of values,
particularly with large starting sets of resistors, the combination
methods now provide generators which put the onus on the calling
function to handle in a memory-appropriate way.

This commit adds the ability to calculate sets of resistors containing parallel combinations
of resistors. It also provides the ability to specify and calculate tolerances of resistor
combinations.