Add FixedWidthTwoLine Reader to guess list

As discussed in #3025 and #3099, it would be useful if tables that are copied
from the screen (where the default print format is something like)
```
col1 col2
---- ----
1.0  2.0
3.0  44.0
```
into a file could be read automatically with the guessing process.

CC: @taldcroft @demitri @embray 

I did not measure the performance, but with this we will surely have a performance hit since this adds a `FixedWidthTwoLine` Reader to the top of the guessing list. This Reader does not have a fast c implementation and it will read only a small fraction of the tables. My impression is that most tables will be read with the `Basic` Reader. If that is true, then we will increase the average read time with guessing by a factor of two. However, since the tables in question would also be read by `Basic` but not in the way we want (string entries like `----` in the first row) there is no way around trying `FixedWidthTwoLine` first.

closes #3099
closes #3025