Added set_params function to all Locator types

This was a bug fix targeted at making the api for Locators more consistent.

In the old behavior, only locators of type MaxNLocator have set_params()

defined, causing its use on any other Locator to throw an AttributeError *(

instance to be as specified within args)*. The fix involves moving set_params()

to the Locator class such that all subtypes will have this function defined.

Since each of the Locator subtype have their own modifiable parameters, a

universal set_params() in Locator isn't ideal. Instead, a default no-operation

function that raises a warning is implemented in Locator. Subtypes extending

Locator will then override with their own implementations. Subtypes that do

not have a need for set_params() will fall back onto their parent's

implementation, which raises a warning as intended.

In the new behavior, all Locator instances will not throw an AttributeError

when set_param() is called. For Locators that do not implement set_params(),

the default implementation in Locator is used.

API Consistency fix within Locators set_params() function

set_params() function, which sets parameters within a Locator type instance,

is now available to all Locator types. The implementation also prevents unsafe

usage by strictly defining the parameters that a user can set.

To use, simply call set_params() on a Locator instance with desired arguments:

::