Add short and long running acceptance tests

[martintoreilly]

Goal

Have short running acceptance tests using real-world networks as part of automatic tests run on CRAN. Have wider set of tests using real-world networks that can be run by those with an interest.

Acceptance criteria

• [ ] Short-running tests written and passing, using quick datasets from issue #3. Tests will be run by CRAN when verifying package.
• [ ] Long-running tests written and passing, using thorough datasets from issue #3. Tests will be excluded from CRAN package verification.

[martintoreilly]

A good non-zero NetEMD test case is: X: mass @ +/-1 (0.5 each) Y: = mass @ 1, 0, -1 with mass distribution to give same variance as X

Add examples with different variance also.