add collect util function
define has_var_dim in terms of has
add isdimension
add remove to util
add var = Var() to coretypes
add ishomogenous
number of dimensions function

I'm not sure that this is how we want to define things
add isfixed predicate
add istabular predicate
all predicates use strings
make collect a function, not a generator
break out collect docstring
homogenous -> homogeneous typo
dshape short circuits in case of DataShape
don't check DataShape, just dshape
isfixed(TypeVar) yields none
make dimensions private
import predicates into datashape
add test_predicates file
Merge pull request #46 from mrocklin/predicates

Predicates