Remove duplicate NETL flow correction method

[dt-woods]

This PR addresses NETL-RIC#260. It removes the duplicate correct_netl_flow_names from natural_gas_upstream.py and updates the method of the same name found in elementaryflows.py. As this method is reference in fix_coal_mining_lci in coal_upstream.py, the results were cross-checked before and after the update

import pandas as pd
from pandas.testing import assert_frame_equal

old_csv = "OLD_ng_lci_2020rev1.csv"  # archived earlier run
new_csv = "ng_lci_2020rev1.csv"
old_df = pd.read_csv(old_csv)
new_df = pd.read_csv(new_csv)

err_msgs = []
idx = 0
while idx < len(new_df) - 1:
    try:
        assert_frame_equal(new_df.loc[idx:,:], old_df.loc[idx:,:], check_dtype=False)
    except Exception as e:
        err_msg = (idx, str(e))
        err_msgs.append(err_msg)
    finally:
        idx += 1

The differences between the two dataframes were only in the 'flow_name' column for five rows:

Index	Error Message
70	'Particulate matter, ≤ 2.5μm' != 'Particulate matter, âÂ\x89¤ 2.5Ã\x8e¼m'
146	'Particulate matter, > 2.5μm and ≤ 10μm' != 'Particulate matter, > 2.5Ã\x8e¼m and âÂ\x89¤ 10Ã\x8e¼m'
151	'Particulate matter, > 10μm' != 'Particulate matter, > 10Ã\x8e¼m'
182	'Total Organic Carbon' != 'Nitrogen'
186	'Phenols' != 'Phenols '

These corrections are based on poor encodings (note the Greek letters) and new mappings.

Global variabless in natural_gas_upstream.py were capitalized to conform with standard.
Minor formatting fixes were applied to conform to PEP8.