broadcast 0.1.7

Consistency fixes:

• In bc.b(), in the case of a result with length zero:
  the result will be of type "raw" if both x and y are raw, and logical otherwise
  (until now, the result was always logical, which is not consistent with the regular case).
• In bc.d():
  the d==, d!= etc. operators now also accept the case where both x and y are integer or logical.
• Most methods and operators now also preserve the comment attribute.

Other changes:

• Added more examples in the reference manual.
• Added more tests.
• Fixed several linguistic errors in the help page of cast_shallow2atomic().