Task: Cleaner printing of units (openmm/openmm#4767)

org: openmm
repo: openmm
pr_number: 4767
issue_title: Cleaner printing of units
issue_body: This changes the default string representation of Quantities to print values with units in a simpler form.  With the current code:

```python
>>> 1.5*nanometers
Quantity(value=1.5, unit=nanometer)
```

With this PR:

```python
>>> 1.5*nanometers
1.5 nm
```
resolved_issues: title: Cleaner printing of units
body: The current string representation of quantities in the library is verbose and not user-friendly. When a quantity is printed, it displays the value and unit in a detailed format, such as `Quantity(value=1.5, unit=nanometer)`. This format is unnecessarily complex and can make the output harder to read, especially when working with a large number of quantities.

The expected behavior is for quantities to be displayed in a cleaner and more concise format, showing only the numeric value followed by the unit symbol. For example, `1.5*nanometers` should be represented as `1.5 nm`. This would improve readability and make the output more intuitive for users.

## IMPORTANT INSTRUCTIONS

1. If you edit a python file, you MUST run 'make PythonInstall' inside the build directory
2. If you edit a c++ file, you MUST run 'make -j4 && make install" inside the build directory

Reference solution:
fix_patch: diff --git a/wrappers/python/openmm/unit/quantity.py b/wrappers/python/openmm/unit/quantity.py
index d93283c7f9..0559c145cd 100644
--- a/wrappers/python/openmm/unit/quantity.py
+++ b/wrappers/python/openmm/unit/quantity.py
@@ -215,8 +215,7 @@ def __str__(self):
     def __repr__(self):
         """
         """
-        return (Quantity.__name__ + '(value=' + repr(self._value) + ', unit=' +
-                str(self.unit) + ')')
+        return str(self)
 
     def format(self, format_spec):
         return format_spec % self._value + ' ' + str(self.unit.get_symbol())