I created a custom module for odoo 8.0 following the instructions found here:
https://www.odoo.com/documentation/8.0/reference/orm.html#inheritance-and-extension
and here:
http://www.pixelite.co.nz/article/adding-additional-fields-using-custom-module-openerp-7/

There is no problem adding new fields when I first install it but I have problems adding more fields - by updating the custom module.
I check through
Settings --> Database Structure -->Fields
and I can find the custom fields added when I first installed the custom modules but none of the fields added later.
Of course, after editing my module's .py file  I always restart the odoo server.
When I reference the new fields in my custom view and try to 'upgrade' through the admin interface, I get an error that
Field 'a_new_field_here' does not exist

What can I do to create the new fields (columns) when I restart the odoo server ?

Here is (a simplified version of) the source code of the custom module per file

__init__.py

import custom_tasks

__openerp__.py

{
    'name': "custom_tasks",
    'version': "0.0.3",
    'author': "George",
    'category': "Tools",
    'depends': ['project'],
    'data': ['task.xml'],
    'demo': [],
    'installable': True,
    'auto_install': False,
    'application': False,
    'description': "Custom tasks module",
}

custom_tasks.py

# vim; set filencoding= utf-8

from openerp import api
from openerp.osv import fields, osv
class custom_tasks(osv.osv):
    _name = "project.task"
    _inherit = "project.task"
    _columns = {
        'name': fields.char('Task Summary', track_visibility='onchange', size=128, required=True, select=True),
        'c_task_attachement': fields.binary('Attachemnt (pdf)'),
        'c_task_numbera': fields.integer('Παράμετρος Α'),
        'c_task_numberb': fields.integer('Παράμετρος Β'),
        'c_task_numberc': fields.integer('Παράμετρος Γ'),
        'date_deadline': fields.datetime('Deadline', select=True, copy=False),
    }
    @api.onchange('project_id') # if these fields are changed, call method
    def _check_change(self):
        self.name = self.project_id.name
custom_tasks()

task.xml

<openerp>
    <data>
        <record model="ir.ui.view" id="custom_task">
            <field name="name">project.task.form</field>
            <field name="model">project.task</field>
            <field name="inherit_id" ref="project.view_task_form2" />
            <field name="arch" type="xml">
                <xpath expr="/form/sheet/group/group[1]/field[@name='project_id']" position="replace"> </xpath>
                <xpath expr="/form/sheet/h1" position="inside">
                    <field name="project_id" class="oe_inline" required="1" />
                </xpath>
                <xpath expr="/form/sheet/h1/field[@name='name']" position="replace">
                    <field name="name" required="1" invisible="1"/>
                </xpath>
                <xpath expr="/form/sheet/group/group[2]/field[@name='date_deadline']" position="replace">
                    <field name="date_deadline" required="1"/>
                </xpath>
                <xpath expr="/form/sheet/group/group[2]/field[@name='categ_ids']" position="replace">
                    <field name="c_task_attachement" widget="binary"/>
                </xpath>
                <xpath expr="/form/sheet/group" position="after">
                    <group>
                        <field name="c_task_numbera" />
                        <field name="c_task_numberb" />
                        <field name="c_task_numberc" />
                    </group>
                </xpath>
            </field>
        </record>
    </data>
</openerp>