Option for issue-126: Convert str to None for location.td in well.plot()

[dcslagel]

This is an alternative solution to pull-request #134 (issue #126 Resolve this issue at the most specific place it occurs in well.py):

Resolve this issue at the most specific place it occurs in well.py::plot():

        elif extents == 'td':
            try:
                # example code
                if isinstance(self.location.td, str):
                    self.location.td = None
                # end example code
                upper, lower = 0, self.location.td
            except:

The benefits of this solution are that it is seems simple and it is right at the location of the issue.

The drawbacks are that it clutters the processing code with data formatting code and that it might (but unlikely to) impact other code using self.location.td in other parts of the code. Initially I tried to stay away from this solution because applying this approach to many problems can fill an application with many local special cases rather than having a consistent way and central place for special case configuration. ... All that said, it is so simple to implement, and if this is a rare special case, it could be a good solution.

Changes:

• Add a self.location.td check for invalid string values in well.plot().
  The value of self.location.td is used to set the 'lower' variable which
  must be either None or a number.
• Include tests for this change in tests/test_location.py

Let me know if this change could be accepted (or rejected) or
needs some additional changes before being approved and merged.

Thank you,
DC

[kwinkunks]

Again, accepting the other PR and trying to fix the root cause. Thank you for the options!