Add test to check platform precision using a string + fix error

[mikemhenry]

Description

Fixes #550

Todos

• Implement feature / fix bug
• Add tests
• Update documentation as needed
• Update changelogNotable points that this PR has either accomplished or will accomplish.

Status

• Ready to go

[mikemhenry]

Okay this was a tricky one!

We did import openmm at the top of utils.py but because we had an openmm import inside the function, it confused python:
https://github.com/choderalab/openmmtools/blob/main/openmmtools/utils.py#L547-L550

See this example

>>> import sys
>>> def foo():
...  print(sys)
...  if True:
...   import sys
... 
>>> foo()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in foo
UnboundLocalError: local variable 'sys' referenced before assignment
>>> 

[jchodera]

Shouldn't we favor local imports (imports inside functions) so that we incur the overhead of importing OpenMM (which sometimes takes several seconds) only when we need it?

[sroet]

Shouldn't we favor local imports (imports inside functions) so that we incur the overhead of importing OpenMM (which sometimes takes several seconds) only when we need it?

(Just my opinion, feel free to ignore)
In general, yes. However, for at least one of the classes defined in that file unit is required to be imported. So you can't really get around the module level openmm import for openmm >= 7.6 (as that is done implicitly when calling from openmm import unit)

@mikemhenry, sorry for not catching this in #528 and thanks for adding a test.

[jchodera]

Great point, @sroet! Looks like there's no way around the import overhead there. This should be good to go then @mikemhenry.

[jchodera]

Thanks!