Update adjust-interval api to use auto-index detection

[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]

After chatting with @rsiddharthan, we decide to make two changes:

• option :also-group-by => :categories
• change fn name adjust-interval => adjust-frequency.

[ezmiller]

Based on this still unfinished conversation with @rsiddharthan, I changed adjust-frequency to return an ungrouped dataset, unless the user overrides that behavior. This undoes the need for the include-columns argument that we were discussing. That said this conversation was unfinished and I hadn't grokked fully what @rsiddharthan was saying. Merging now anyway because lots of time has passed and I think this is a reasonable approach for now. We can always change course later.