Feature/issue 865 lkj cov deprecate

#### Summary:

Adds deprecation warning for uses of `lkj_cov_log` function or `lkj_cov` sampling distribution.

#### Intended Effect:

See summary.

#### How to Verify:

There's a new parser test with two new models that check the messages are being printed.

#### Side Effects:

None.

#### Documentation:

The documentation for lkj_cov is being removed.

#### Reviewer Suggestions:

Anyone.