Clean Columns header

Clean_header will replace space with underscore and convert upper case letter to lower case

def clean_headers(headers):
    newheaders = []

    for i in headers:
        l = i.strip()
        l = l.lower()
        l = l.replace(' ','_')
        newheaders.append(l)

    return newheaders

df.columns = clean_headers(df.columns)