v0.4.0

This is the v0.4.0 release of Topsoil. The major changes (as far as I
can recall) are as follows:

- Chart plot area is now properly clipped
- Wetherill line no longer extends past t=0 or t=4.6e9
- Configuration panel formatting improvements
- More predictable choice boxes
- Topsoil automatically adds a fifth column of zeros for uncorralated data
- Internal improvements