Nblast everything

Implements #7. Now we can put in all types of combinations of ``dotprops`` and ``neurons`` in single and ``neuronlist`` form. The speed of calculation of ``dotprops`` against ``dotprops`` (the most common use case) is not altered by these changes:
* Before:

    ```r
    > microbenchmark(foo <- nblast(kcs20, kcs20))
    Unit: milliseconds
                            expr      min      lq   median       uq      max neval
     foo <- nblast(kcs20, kcs20) 515.0705 523.399 527.6887 539.2831 731.1246   100
    ```
* After:

    ```r
    > microbenchmark(foo <- nblast(kcs20, kcs20))
    Unit: milliseconds
                            expr      min      lq   median       uq      max neval
     foo <- nblast(kcs20, kcs20) 517.9713 525.908 530.4511 534.9159 653.9733   100
    ```