Adds localMaxima function
Adds explanation for Local Maxima exercise
Removes unneeded map from everyNth

After implementing localMaxima, it became clear that it was sufficient to use a list comprehension to write everyNth. Explicitly using (x,y) as the variable in the input set keeps everything self-contained and gets rid of the awkward map, fst, snd construction I had.
Adds a count function

count takes an integer n and list of integers l and returns the number of occurrences of n in l.