add register class methods

[sgrava]

Add class methods to Register to create special registers:

• 1. Register.circle
  issue: add circle class method to Register #407
  PR:
• 2. Register.rectangular
  issue: add rectangular class method to Register #408
  PR:
• 3. Register.triangular
  issue: add triangular class method to Register #409
  PR:
• 4. Register.square
  issue: add square class method to Register #410
  PR:
• 5. Register.line
  issue: add line class method to Register #411
  PR:
• 6. Register.hexagonal
• 7. Register.heavy_hexagonal (to be discussed, maybe low priority)

Description

Register.circle, Register.rectangular and Register.triangular are the simplest primitives to start.
All methods:

• should accept a spacing (or a better name) kwarg that sets the minimum pairwise distance (Default to 1.0).
• return coordinates, centered around the origin (0,0).

Naming

Methods that tile a repeating unit cell (rectangular, triangular, hexagonal, heavy_hexagonal) use the adjectival form, matching the equivalent DataGraph methods. Methods that describe the register's overall outline (circle, line, square) keep the noun form.

Context

These methods should cover already available methods in DataGraph, but specialized here to just return a register with coordinates.
Next step will be to remove positions stored in those graph methods, achieving pure graph output.
Users that were using such methods just for positions will need to be informed about this change.

Acceptance criteria

all methods should

• test simple layout with explicit coords
• test minimum distance is given by spacing