Add interdigital capacitor

[nikosavola]

This PR implements an interdigital capacitor component with parametrisation for

• amount of fingers
  • length
  • gap
  • thickness

Also, using finger_length=0, one gets a plate capacitor

@joamatab, could you check if there's anything missing in the component definition here?

Part of #610

[sourcery-ai]

Sourcery Code Quality Report

Merging this PR leaves code quality unchanged.

Quality metrics	Before	After	Change
Complexity	3.00 ⭐	3.00 ⭐	0.00
Method Length	1937.00 ⛔	1934.00 ⛔	-3.00 👍
Working memory	36.00 ⛔	36.00 ⛔	0.00
Quality	30.68% 😞	30.68% 😞	0.00%

Other metrics	Before	After	Change
Lines	560	556	-4

Changed files	Quality Before	Quality After	Quality Change
gdsfactory/components/__init__.py	30.68% 😞	30.68% 😞	0.00%

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Working Memory	Quality	Recommendation

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[joamatab]

Looks great, thank you Niko!