Create Air-Core Inductor Functions

[engineerjoe440]

We've got one Air Core Inductor formula, but it would be beneficial to have a few others, namely we need to:

• calculate required length
• calculate desired diameter
• calculate desired number of turns

We should use the formulas called out here and make three formulas:

• air_core_required_length
• air_core_required_diameter
• air_core_required_num_turns