Add toInteger() basic EL function

The set of Basic EL Functions described in the docs section 4.2.2 should be extended to include a basic function called toInteger(). This is useful so that we can define parameters in the properties files that we can easily convert from String to Integer to pass into functions that accept int as input parameters.

For example, many of the Dataset or Date EL functions take 'int' as input, such as coord:dateOffset [1], coord:offset [2] and certainly numerous others.

Currently, any parameter defined in the job properties file is considered a String, and can not be used out of the box as an input parameter to these functions.

Having a simple toInteger() function can help simplify this.

[1] https://oozie.apache.org/docs/4.3.0/CoordinatorFunctionalSpec.html#a6.9.1._coord:dateOffsetString_baseDate_int_instance_String_timeUnit_EL_Function
[2] https://oozie.apache.org/docs/4.3.0/CoordinatorFunctionalSpec.html#a6.6.2._coord:offsetint_n_String_timeUnit_EL_Function_for_Synchronous_Datasets