Remove double word typos.

Remove double word typos.

This commit removes typos consisting of an accidentally repeated
word. Attached below is the python script I used to find them.

It is worth noting that there is a large false positive rate here, for
example, in tria_accessor.h we have the sentence

/**
 * [...] As a
 * consequence, it exists in the mesh to ensure that each processor has all
 * coarse mesh cells and that the 2:1 ratio of neighboring cells is
 * maintained, but it is not one of the cells we should work on on the
 * current processor. [...]
 */

Here 'on on' is correct.

script:

import sys

SKIP = ["//", "*", "}", "|", "};", ">", "\"", "|", "/", "numbers::invalid_unsigned_int,", "std::string,", "int,"]

with open(sys.argv[1], 'r') as handle:
    previous_line = ""
    for line_n, line in enumerate(handle):
        line = line.strip()
        previous_words = previous_line.split()
        words = line.split()
        # ignore comment blocks '*' and comment starts '//' at the beginning of
        # each line.
        if len(words) == 0:
            continue
        if words[0] in ["*", "//"]:
            words = words[1:]

        # See if the last word on the previous line is equal to the first word
        # on the current line.
        if len(previous_words) != 0:
            if words[0] not in SKIP and previous_words[-1] == words[0]:
                print(sys.argv[1] + ":{}: {}".format(line_n + 1, previous_line))
                print(sys.argv[1] + ":{}: {}".format(line_n + 2, line))
        previous_line = line

        for left_word, right_word in zip(words[:-1], words[1:]):
            if left_word == right_word and left_word not in SKIP:
                print(sys.argv[1] + ":{}: {}".format(line_n + 1, line))