Fix lascheck report of invalid data index channel

[dcslagel]

Description:

Reorder curve_df so that a valid Las2 index channel is the first data
column.

Running a dlis-to-las generated las file through lascheck reports an 'Invalid index
mnemonic'The only valid mnemonics for the index channel are DEPT, DEPTH,
TIME, or INDEX.’]. This is for Las 2.0 spec.

This may help other tools read the Dlis-to-Las las files.

To reproduce:

pip install lascheck

import lascheck
myfile = 'dlis-to-las.las'
print('-------------------------------------------')
las = lascheck.read(myfile)
print('-------------------------------------------')
print(myfile, las.check_conformity())
print('-------------------------------------------')
print(las.get_non_conformities())

Result

['Invalid index mnemonic. The only valid mnemonics for the index channel are DEPT, DEPTH, TIME, or INDEX.']

Test Results

All tests pass

Let me know if this change could be accepted (or rejected) or
needs some additional changes before being approved and merged.

Thank you,
DC