Update nwis client to canonical format

[jarq6c]

nwis_client returns a value_date column. We will add a value_time_label="value_date" option to continue this behavior.

Update 1: Add value_time_label="value_date" to __init__ that will raise a deprecation warning. Default behavior will return value_date. value_time_label="value_time" will return value_time.

Update 2: Default behavior is value_time_label="value_time". Warn if this option is not explicitly set by user.

Update 3: Default behavior is value_time_label="value_time" with no warning.

[jarq6c]

DeprecationWarning is suppresed by default. Changing to UserWarning.

[jarq6c]

Update 1 implemented in #118

[aaraney]

I think this might need to be reopened, @jarq6c. It appears that #185 did change the default value of value_time_label to "value_time" (Update 3.). However, value_time_label still throws a UserWarning is a user passes None to value_time_label and then sets value_time_label to "value_date" not the, now canonical, "value_time".

[jarq6c]

I think this might need to be reopened, @jarq6c. It appears that #185 did change the default value of value_time_label to "value_time" (Update 3.). However, value_time_label still throws a UserWarning is a user passes None to value_time_label and then sets value_time_label to "value_date" not the, now canonical, "value_time".

Ah got me! I forgot about removing the warning!