dconv no longer uses global instances of StringToDoubleConverter and

[das-intensity]

…DoubleToStringConverter

Fixes #420
Closes #420 (nice)

Changes proposed in this pull request:

• Stop using global instances of StringToDoubleConverter and DoubleToStringConverter across threads

Based on my python knowledge, there should be no reason that this change is necessary, however

1. It fixes a (hard to debug/reproduce) issue that multiple users have had
2. It doesn't cause any harm, and I would argue is more correct regardless

Thanks to @yechiel-optimalq for his detailed issue report, and @klimermana for helping debug and develop this fix

If this is accepted, please also backport to 3.2 as some people are stuck using python3.5 (despite it being EOL).

[das-intensity]

@hugovk do we need anything else for this PR?

[hugovk]

Let's get this merged, thanks!