Add ff as a suggested package
check that we can give an ff matrix as input to fc_subscoremat
two more tests

* which pass
factor out a function to extract diagonal terms from fc_subscoremat

* getting diagonal terms out of an ff matrix is actually fast like this
* this is also the fastest way to acc ess a big.matrix
DOC: also mention ff matrices
Add ability to download and attach ff objects + test
Add ability for fc_attach_bigmat to handle ff objects
fix import error in diagonal()
check that original fc_nblast and fc_subscoremat agree
correct handling of queries when the result has a singleton dimension

* this has been a general problem of fc_subscoremat
fix bug in extracting diagonal of bigmatrix objects
reimplent fc_nblast using fc_subscoremat
reimplement fc_sub_distmat using fc_subscoremat

* note that results will not be identical because fc_sub_distmat returns 
  matrices with row/column ordering according to the underlying distance
  matrix
* whereas fc_subscoremat respects the requested ordering
* this will result in different dendrograms since in many cases dendrogram
  ordering is driven by input order not anything calculated from the data
more specific clustering test with less external dependencies
Merge remote-tracking branch 'jgit/ff' into feature/ff

* jgit/ff:
  Add ability for fc_attach_bigmat to handle ff objects
  Add ability to download and attach ff objects + test
import is.ff from ff (now import not suggests)
correct path in downloaded ff objects

* and test this
load ff library in tests
Remove download KC scores from tests
Exclude inst/extdata from build