add utility module for unit conversions

Adds mxcubecore.utils.units module, so there is a common place
for unit converson utility functions.

Adds some initial function to convert between time, energy, length
and current units.

CONTRIBUTING.md

@@ -121,6 +121,10 @@ code base
  * Pixels are to be used for beam location (center)
  * Datetime YYYY-MM-DD HH:MM:SS(.ff) ,possibly with hundreds of seconds (ff), and with 24 hour clock.
 
+When writing code that converts between different units,
+it is recommended to use utility functions from {py:mod}`mxcubecore.utils.units` module.
+This will to aid in the readability of the code.
+
 #### Value update signals/callbacks
 The "valueChanged" and "stateChanged" signals should be used when a HardwareObjects value or state
 has been changed. Defined in for instance the base class [HardwareObject](https://github.com/mxcube/HardwareRepository/blob/ea8369ab2c08dbe539fd92ffee18fd21bb3a81b8/BaseHardwareObjects.py#L666), [AbstractMotor](https://github.com/mxcube/HardwareRepository/blob/master/HardwareObjects/abstract/AbstractMotor.py) and

mxcubecore/utils/units.py

@@ -0,0 +1,71 @@
+"""
+Utility functions for converting between different units.
+"""
+
+#
+# time units
+#
+
+
+def us_to_sec(us: float) -> float:
+    """
+    convert microseconds (μs) to seconds
+    """
+    return us / 1_000_000.0
+
+
+def sec_to_us(sec: float) -> float:
+    """
+    convert seconds to microseconds (μs)
+    """
+    return sec * 1_000_000.0
+
+
+def sec_to_hour(sec: float) -> float:
+    """
+    convert seconds to hours
+    """
+    return sec / (60 * 60)
+
+
+#
+# energy units
+#
+
+
+def ev_to_kev(ev: float) -> float:
+    """
+    convert eV value to KeV value
+    """
+    return ev / 1000.0
+
+
+#
+# length units
+#
+
+
+def meter_to_mm(meters: float) -> float:
+    """
+    convert meters to millimeters (mm)
+    """
+    return meters * 1000.0
+
+
+def mm_to_meter(millimeters: float) -> float:
+    """
+    convert millimeters (mm) to meters
+    """
+    return millimeters / 1000.0
+
+
+#
+# current units
+#
+
+
+def A_to_mA(amp: float) -> float:
+    """
+    convert Ampere (A) to milli Ampere (mA)
+    """
+    return amp * 1000

test/pytest/test_utils_units.py

@@ -0,0 +1,45 @@
+from math import isclose
+from mxcubecore.utils.units import (
+    us_to_sec,
+    sec_to_us,
+    sec_to_hour,
+    ev_to_kev,
+    meter_to_mm,
+    mm_to_meter,
+    A_to_mA,
+)
+
+
+def test_us_to_sec():
+    assert isclose(us_to_sec(500_000), 0.5)
+    assert isclose(us_to_sec(123.4), 0.0001234)
+
+
+def test_sec_to_us():
+    assert isclose(sec_to_us(2), 2_000_000.0)
+    assert isclose(sec_to_us(0.42), 420_000.0)
+
+
+def test_sec_to_hour():
+    assert isclose(sec_to_hour(3800), 1.056, abs_tol=0.001)
+    assert isclose(sec_to_hour(1800.0), 0.5)
+
+
+def test_ev_to_kev():
+    assert isclose(ev_to_kev(12000), 12.0)
+    assert isclose(ev_to_kev(10.5), 0.0105)
+
+
+def test_meter_to_mm():
+    assert isclose(meter_to_mm(10), 10_000.0)
+    assert isclose(meter_to_mm(0.5214), 521.4)
+
+
+def test_mm_to_meter():
+    assert isclose(mm_to_meter(1200), 1.2)
+    assert isclose(mm_to_meter(10.5), 0.0105)
+
+
+def test_A_to_mA():
+    assert isclose(A_to_mA(2), 2000.0)
+    assert isclose(A_to_mA(0.3921), 392.1)