Switch default scoremats to explicitly check option
refactor fc_subscoremat to use nat.nblast::sub_score_mat

* closes #22
remove internal diagonal function

* now in nat.nblast
move test for diagonal() to nat.nblast

* since the function has been removed
fix: make sure hclustfc attaches scoremat if reqd

* was not attaching the scoremat defined in options("flycircuit.scoremat")
  but assuming someone else had already done this.
test that we can cluster using scoremat in options
Merge branch 'fix/cached-nblast'

* fix/cached-nblast:
  test that we can cluster using scoremat in options
  fix: make sure hclustfc attaches scoremat if reqd
  move test for diagonal() to nat.nblast
  remove internal diagonal function
  refactor fc_subscoremat to use nat.nblast::sub_score_mat
  Switch default scoremats to explicitly check option
* closes #21
dev: git ignore figure directory

* i.e. the one produced by vignettes
Bump version to 0.5.1