BUG: Ensure that the min/max values can be reached in ctkDoubleSlider and ctkDoubleRangeSlider

[lassoan]

The minimum and maximum values were not always reachable in ctkDoubleSlider and ctkDoubleRangeSlider widgets when moved the slider to the minimum or maximum position.

For example:

lo = -1024
hi = 2000
step = (hi - lo) / 1000.

thresholdSlider = ctk.ctkRangeWidget()
thresholdSlider.spinBoxAlignment = qt.Qt.AlignTop
thresholdSlider.setRange(lo, hi)
thresholdSlider.singleStep = step
thresholdSlider.show()
# Impossible to reach 2000.0

slider = ctk.ctkDoubleSlider()
slider.minimum = lo
slider.maximum = hi
slider.singleStep = step
slider.show()
slider.connect("valueChanged(double)", lambda v: print(v))
# Impossible to reach 2000.0

The problem was that the integer slider range was determined by rounding, which could result in the integer slider range not covering the entire value range.
Fixed the problem by extending the integer slider range when it did not fully cover the value range. There are no regressions in tests.

[lassoan]

Thank you @jcfr, I've merged the changes you suggested. Please check it out and approve if looks good.

[jcfr]

$ LD_LIBRARY_PATH=/home/jcfr/Support/Qt/5.15.2/gcc_64/lib/ hctest -R "^ctkRange|ctkDoubleRange" -j5
Test project /home/jcfr/Projects/CTK-Qt5-build/CTK-build
      Start  63: ctkDoubleRangeSliderTest
      Start  64: ctkDoubleRangeSliderTest1
      Start  65: ctkDoubleRangeSliderTest2
      Start  66: ctkDoubleRangeSliderValueProxyTest
      Start 114: ctkRangeSliderTest
 1/12 Test  #66: ctkDoubleRangeSliderValueProxyTest ...............   Passed    0.26 sec
      Start 115: ctkRangeSliderTest1
 2/12 Test  #63: ctkDoubleRangeSliderTest .........................   Passed    0.46 sec
      Start 116: ctkRangeWidgetTest
 3/12 Test  #65: ctkDoubleRangeSliderTest2 ........................   Passed    0.55 sec
      Start 117: ctkRangeWidgetTest1
 4/12 Test  #64: ctkDoubleRangeSliderTest1 ........................   Passed    0.59 sec
      Start 118: ctkRangeWidgetValueProxyTest
 5/12 Test #115: ctkRangeSliderTest1 ..............................   Passed    0.52 sec
      Start 160: ctkDoubleRangeSliderEventTranslatorPlayerTest1
 6/12 Test #118: ctkRangeWidgetValueProxyTest .....................   Passed    0.33 sec
      Start 172: ctkRangeSliderEventTranslatorPlayerTest1
 7/12 Test #116: ctkRangeWidgetTest ...............................   Passed    0.51 sec
      Start 173: ctkRangeWidgetEventTranslatorPlayerTest1
 8/12 Test #117: ctkRangeWidgetTest1 ..............................   Passed    0.63 sec
 9/12 Test #114: ctkRangeSliderTest ...............................   Passed    1.88 sec
10/12 Test #173: ctkRangeWidgetEventTranslatorPlayerTest1 .........   Passed    8.90 sec
11/12 Test #172: ctkRangeSliderEventTranslatorPlayerTest1 .........   Passed   27.36 sec
12/12 Test #160: ctkDoubleRangeSliderEventTranslatorPlayerTest1 ...   Passed   28.26 sec