Don't Use Asserts For Runtime Errors

[riggsd]

While pyshp is generally good about raising appropriate exceptions, it currently uses assert() in two places:

• Possibly corrupted DBF header
• User attempting to write larger value than a field allows for

Not only are these both examples of cases where assert() isn't appropriate, they're cases that could lead to data corruption if uncaught... and when CPython is invoked with -O or -OO, assert() conditions are never raised.

This patch raises a descriptive ShapefileException in these two cases.

[GeospatialPython]

Good call on this one. The dbf sections of the library are based on this simple recipe from the Python Cookbook: http://code.activestate.com/recipes/362715-dbf-reader-and-writer/

In fact I'm strongly considering replacing dbf handling with something like dbfpy. However I'd also like this library to stay a simple one-file library and not turn into a "shapefile framework".