Add and use isArray() and isString() methods to Core Loader.

[kaladay]

Provide PHP enumerations that can perform the appropriate checks themselves.

I considered implementing the logic within the CoreFunctions but opted to keep all of the verification logic self-contained within the enumerations themselves.

This allows for the reduction of the extra type parameter.
The enumeration knows its own type and can perform the appropriate check.

This is designed to ensure that all appropriate checks are performed in order to guarantee that a boolean is returned.
The caller need only perform the is() or the in() check using the appropriate configuration type.

Only the most simplistic types are chosen.
If additional php is_*() types are needed, then enumerations may be added.

The special NULL supporting types are done to reduce the needed checks should something care to check if the type is either NULL or a given type.
This doesn't increase the code complexity much at all and the use should reduce code complexity.
The NULL checks need to be performed anyway.

A specific type ANY can be used to test if a configuration type exists as is NULL.
The in() method allows for array key existence checks and then a type check on some child property.
This should allow for shallow array value type checks.

The non-$config based isset() and is_string() checks are reverted to previous behavior.

This should allow for the configuration checks to be performed anywhere without warnings or problems.

Original Description and Design

These allow for cleaner existence checks on arrays without PHP warnings or problems.
An isset() must be called to safely perform something like is_string() on a value at some array index.
Adding an isset() inline can make the code messier so move this logic out into two methods:

• isArray().
• isString().

Update all code in CoreLoader class to utilize these methods.

This approach was changed to the enum functions approach based on this comment:

• Add and use isArray() and isString() methods to Core Loader. #98 (review)

[jsavell]

I think it makes sense to add a hasConfiguration($property, $type=string(default)|array|bool|integer|etc) or similarly named method to the CoreFunctions:
https://github.com/TAMULib/Pipit/blob/master/src/Pipit/Lib/CoreFunctions.php

And provide a wrapper method via CoreObject:
https://github.com/TAMULib/Pipit/blob/master/src/Pipit/Classes/CoreObject.php

That will provide a standardized means of checking the config values to both the library and clients.