Document that block_size must be an odd integer

[HaleySchuhl]

Is your feature request related to a problem? Please describe.
The updated pcv.transform.detect_color_card function has kwargs that include block_size. An even number for block_size will throw a long and strange cv2 error, see below, so best there's mention in the documentation page transform_detect_color_card.md that block_size must be an odd integer.

Describe the solution you'd like
mention in the documentation page transform_detect_color_card.md that block_size must be an odd integer

Describe alternatives you've considered
add logic to the pcv.transform.detect_color_card function that checks this and throws a pcv error message rather than getting to the cv2 part.

Additional context

error                                     Traceback (most recent call last)
Cell In[34], line 2
      1 # First, detect the color card.
----> 2 card_mask = pcv.transform.detect_color_card(rgb_img=crop_img, adaptive_method=0, radius=15, block_size=10)
      4 # Print average chip size, value is stored in outputs.observations
      5 #print(pcv.outputs.observations['B73']['median_color_chip_size']['value'])
File ~/.local/lib/python3.9/site-packages/plantcv/plantcv/transform/detect_color_card.py:69, in detect_color_card(rgb_img, label, **kwargs)
     67 imgray = cv2.cvtColor(rgb_img, cv2.COLOR_BGR2GRAY)
     68 gaussian = cv2.GaussianBlur(imgray, (11, 11), 0)
---> 69 thresh = cv2.adaptiveThreshold(gaussian, 255, adaptive_method,
     70                                cv2.THRESH_BINARY_INV, block_size, 2)
     71 contours, _ = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
     73 # Filter contours, keep only square-shaped ones
error: OpenCV(4.9.0) /io/opencv/modules/imgproc/src/thresh.cpp:1675: error: (-215:Assertion failed) blockSize % 2 == 1 && blockSize > 1 in function 'adaptiveThreshold'error                                     Traceback (most recent call last)
Cell In[34], line 2
      1 # First, detect the color card.
----> 2 card_mask = pcv.transform.detect_color_card(rgb_img=crop_img, adaptive_method=0, radius=15, block_size=10)
      4 # Print average chip size, value is stored in outputs.observations
      5 #print(pcv.outputs.observations['B73']['median_color_chip_size']['value'])
File ~/.local/lib/python3.9/site-packages/plantcv/plantcv/transform/detect_color_card.py:69, in detect_color_card(rgb_img, label, **kwargs)
     67 imgray = cv2.cvtColor(rgb_img, cv2.COLOR_BGR2GRAY)
     68 gaussian = cv2.GaussianBlur(imgray, (11, 11), 0)
---> 69 thresh = cv2.adaptiveThreshold(gaussian, 255, adaptive_method,
     70                                cv2.THRESH_BINARY_INV, block_size, 2)
     71 contours, _ = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
     73 # Filter contours, keep only square-shaped ones
error: OpenCV(4.9.0) /io/opencv/modules/imgproc/src/thresh.cpp:1675: error: (-215:Assertion failed) blockSize % 2 == 1 && blockSize > 1 in function 'adaptiveThreshold'