Astroua/CARTAvis#179 upper and lower limits of colormap should not be quantized to real pixel values

[kswang1029]

Astroua/CARTAvis#179
To investigate this issue:

1. open HD163296_CO_2_1.image.mom0
2. We should see lower and upper limits in colormap are 0.00333363 and 1.99905, respectively
3. now change 1.99905 to 3.5, we should see no change. This should be fixed. Users should have freedom to define any number, not restrict to the closest pixel value
4. now change 1.99905 to 1.5, we will see 1.49783, since the closest number is 1.49783 (we need to check if there is dedicated calculations for this everytime a user modifies the values)