Add factory methods and unit tests

[olexandr-konovalov]

@harvard-tham unless these two commits are related or you plan to merge this into rewrite really quickly avoid piling up commits in the same pull request. Imagine that the earlier commit looks good and can be merged, but the later one has some problems and needs more work. Then this would block the earlier commit appearing in rewrite where it may be of an interest to other collaborators. Making two separate commits in parallel or one after another from the start may be is easier.

[olexandr-konovalov]

@harvard-tham also, the title no longer matches the content - you can edit the title and PR description to reflect that.

[harvard-tham]

I have updated the unit tests in tst/. They all pass on my machine, is there anyway to merge these unit tests too?

[harvard-tham]

@harvard-tham unless these two commits are related or you plan to merge this into rewrite really quickly avoid piling up commits in the same pull request. Imagine that the earlier commit looks good and can be merged, but the later one has some problems and needs more work. Then this would block the earlier commit appearing in rewrite where it may be of an interest to other collaborators. Making two separate commits in parallel or one after another from the start may be is easier.

Ok I will make separate pull requests for different features from now on

[olexandr-konovalov]

@harvard-tham we possibly don't have to call files with the testunit_ prefix - those in tst will be processed automatically. I only have used that prefix to distinguish it from test_csv in the root directory used for the script based test.