Transfer file validate() from reduce.py to service

[Pasarus]

Issue raised by: developer

What?
Currently in all reduce.py we have:

def validate(file, dir):
    """
    Function that validates if a file and/or directory exist. If not a
    RunTimeError is raised which is picked up by Autoreduction.

    :param file: full path of data file. Provide empty string to ignore
    :type file: str
    :param dir: full path of a directory. Provide empty string to ignore
    :type dir: str
    """
    print("Running validation")
    if file:
        if not os.path.isfile(file):
            raise RuntimeError("Unable to find file: {}".format(file))
    if dir:
        if not os.path.isdir(dir):
            raise RuntimeError("Unable to find directory: {}".format(dir))
    print("Validation successful")

and this method is called in main to check input_file and output_dir exist and return error messages back to users which can be presented in Webapp.

With this issue:

• move this validate step to sit within Autoreduction service, work as before and with furhter potential benefit not starting reduction in the first place if this validation fails (Dimitar)
• update http://reduce.isis.cclrc.ac.uk/help/ accordingly
• delete (I suggest rather than update) example_reduce.py and example_reduce_vars.py in https://github.com/ISISScientificComputing/autoreduce/tree/master/WebApp. I suggest delete these because no longer any point of these I believe with this info in http://reduce.isis.cclrc.ac.uk/help/

Follow up after validation of this work:

• contact the instrument scientists that are known maintainers of reduce.py and inform of this simplication change to their scripts, and this include Pascal, Rob and Duc
• remove validate() code from all reduce.py files including for production instruments

How to test the issue is resolved
unit testing please