Xsection models - some issues

[mbakker7]

In Xsection models I encountered two errors:

1. Create a SimpleModel and then add an Inhom from -np.inf to +np.inf with, for example, h* or recharge. This does not work as those elements try to determine the aquifer they are in at the center. But there is not center between -np.inf and +np.inf.

2. You cannot put a River1D element exactly on the edge of an inhomogeneity. Not sure how to fix that elegantly. Nudging it a bit probably works but is also a bit strange. The nudging should at least be larger than the placement of controlpoints on the left and right sides of an inhomogeneity. I ran into this for timflow.transient. Didn't test it for timflow.steady yet.

[dbrakenhoff]

Alright, the first issue is fixed in #107.

For the second, I added a new method check_elements() to the ModelXsection class that checks the element positions after initialization. I now check for any elements on the boundaries between Xsections and raise an informative error if there are any. (I currently assume there are no elements that would be allowed to be placed there.)