Ethan/update adjust interval take2

[ezmiller]

Note: This PR should go in after #45, if it is approved.

Goal / Problem

The adjust-interval function had an API that was designed for when we were anticipating not knowing yet how we would get the index. We know that now because we have the index on the column in tmd.

Proposed Solution

This PR does the following to the adjust-interval API:

• The index column is now "auto" detected via the get-column-name-or-error fn added in #45;
• The idea of "keys" (taken from R) that was previously there with the idea that we might add mimick it here has been turned into a hopefully explicit option :also-group-by in lieu of us supporting the idea of keys officially (#44);
• The column name of the index column that this fn adjust can for now not be changed. This is because we currently do not yet maintain state of the :index meta carefully enough. We had a solution for this that we need to re-introduce (i.e. see @daslu's #5 )
• The docstring has been updated.

So usage now is like this:

(def data 
  (tablecloth/dataset {:A [#time/date "1970-01-01" #time/date "1970-01-02"
                             #time/date "1970-02-01" #time/date "1970-02-02"]
                         :B ["foo" "foo" "foo" "bar"]
                         :C (range 4)}))

data
;; => _unnamed [4 3]:
;;    |         :A |  :B | :C |
;;    |------------|-----|---:|
;;    | 1970-01-01 | foo |  0 |
;;    | 1970-01-02 | foo |  1 |
;;    | 1970-02-01 | foo |  2 |
;;    | 1970-02-02 | bar |  3 |

(-> data
  (adjust-interval tablecloth.time.api/->months-end)
  (tablecloth.api/ungroup)
;; => _unnamed [4 3]:
;;    |         :A |  :B | :C |
;;    |------------|-----|---:|
;;    | 1970-01-31 | foo |  0 |
;;    | 1970-01-31 | foo |  1 |
;;    | 1970-02-28 | foo |  2 |
;;    | 1970-02-28 | bar |  3 |

(-> data
  (adjust-interval tablecloth.time.api/->months-end)
  (tablecloth.api/aggregate (comp tech.v3.datatype.functional/mean :C))
;; => _unnamed [2 2]:
;;    | :summary |         :A |
;;    |---------:|------------|
;;    |      0.5 | 1970-01-31 |
;;    |      2.5 | 1970-02-28 |

[ezmiller]

Messed up this branch, so closing and will start new one...