`assert_near` behaves inconsistenly with `str` values

[bonjourmauko]

Hi there!

I really enjoy OpenFisca, but I recently encountered an issue.

Here is what I did:

Write a formula declared as str.

Here is what I expected to happen:

To take the value as a numpy.dtype[str_].

Here is what actually happened:

It does not. Instead, it tries to run numexpr.evaluate over it.

Here is data (or links to it) that can help you reproduce this issue:

ba1ff45