Add importStep class method to Shape

[DavidMStraub]

When using purely functional CadQuery, importStep is a bit awkward as it returns a Workplane, so val() or vals() has to be used to return to the world of Shapes.

Since Shape already has other import/export class methods, I thought it fitting to add an importStep method. Obviously, it needs to strictly return a Shape, so if the STEP doesn't contain one, it raises a ValueError.

Before:

import cadquery as cq

shape = cq.importers.importStep("plate.step").val()

After:

import cadquery as cq

shape = cq.Shape.importStep("plate.step")

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.79%. Comparing base (f69500e) to head (be774db).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2061   +/-   ##
=======================================
  Coverage   95.78%   95.79%           
=======================================
  Files          30       30           
  Lines        9427     9439   +12     
  Branches     1404     1407    +3     
=======================================
+ Hits         9030     9042   +12     
  Misses        242      242           
  Partials      155      155           

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