Add filtering options to get_dataset

[Moohan]

I needed this for another project, so had a look and realised it was quite simple. This closes #8

I added some tests and spent a bit of time improving the error messages around the checking of the args (parse_col_select() and parse_row_filter())

[Moohan]

Looks like checks are failing, I need to make the row_filter test less strict to accept a named vector as well as a list (in fact at the moment it only checks for a list but really it needs to quite specifically be a 'depth=1' list with names...

[Moohan]

I should have stopped with just the row and column filters on get_dataset()!

I think everything is working now - all the issues I ended up having were to do with me trying to improve / make stricter the parse_* functions.

[csillasch]

Looks good!
Am wondering about the error handling now that weve switched to retry - for example when trying to select a column that doesnt exist (or is mistyped) I think the call should fail and return the error message immediately rather than retrying. What do you think?