ENH: improved condition selection method so that variables can be used as well

Beforehand the Builder `Select Rows` param was code but then used as a
string :-/ That meant you couldn't provide a variable to it using $.

Now the parameter is a string, but can be replaced by $ to give a
variable name instead. Then the importConditions can handle either a
string or a list/array or slice:

	[1,2,5] or "1,2,5" or "[1,2,5]"
	"1:10:2" or slice(1,8,2) but not 1:8:2 (that isn't a valid
variable
	random(5)*10
And any of these are now fine in Builder::

	1,2,5 or $[1,2,5]
	1:10:2 or $slice(1,8,2) but not $1:8:2
	$random(5)*10