fix calculation of color index

[MxMartin]

The current code returns erroneous colors.
To make it clear I took the current implementation and put it into a separate function

def new_implementation(value):
    if value <= index[0]:
        return 0
    if value >= index[-1]:
        return len(index) - 1
    return next(i for i, v in enumerate(color_map.index) if v > value) -1

def current_implementation(value):
    if value <= index[0]:
        return 0
    if value >= index[-1]:
        return len(index) - 1
    return len([u for u in index if u < value]) - 1

index = [1, 2, 4]
colors = ["black", 'purple', 'red']

print("value", "current implementation", "new implementation")
for i in range(-2, 5):
    print(i, "\t", current_implementation(i), "\t", new_implementation(i)) 

returns:

value current_implementation new_implementation
-2 	 0 	 0
-1 	 0 	 0
0 	 0 	 0
1 	 0 	 0
2 	 0 	 1
3 	 1 	 1
4 	 2 	 2
5 	 2 	 2

you will notice that for value 2 the current implementation would return color 0, even though value 2 is at index 1 in index.

Or for

index = [2, 4, 5]

this returns

value current_implementation new_implementation
-2 	 0 	 0
-1 	 0 	 0
0 	 0 	 0
1 	 0 	 0
2 	 0 	 0
3 	 0 	 0
4 	 0 	 1
5 	 2 	 2
6 	 2 	 2
7 	 2 	 2

i.e. for index 4 we get color 0 and for index 5 color 2. Color 1 is never returned though it should be returned vor value 4

Last but not least I added a check on init to make sure the index is in order

[Conengmo]

Superseded by #141