:1 :2 should only be appended to mnemonics when using LASFile.read

[kinverarity1]

Describe the bug
Confusing overloaded behaviour with LASFile.__setitem__ which creates duplicate mnemonics with appended :1, :2 when trying to update curves.

To Reproduce

>>> import lasio.examples
>>> las = lasio.examples.open("sample.las")
>>> print(las.curves)
Mnemonic  Unit  Value  Description
--------  ----  -----  -----------
DEPT      M            1  DEPTH
DT        US/M         2  SONIC TRANSIT TIME
RHOB      K/M3         3  BULK DENSITY
NPHI      V/V          4   NEUTRON POROSITY
SFLU      OHMM         5  RXO RESISTIVITY
SFLA      OHMM         6  SHALLOW RESISTIVITY
ILM       OHMM         7  MEDIUM RESISTIVITY
ILD       OHMM         8  DEEP RESISTIVITY
>>> las["ILD_MSM"] = 100 / las["ILD"]
>>> print(las.curves)
Mnemonic  Unit  Value  Description
--------  ----  -----  -----------
DEPT      M            1  DEPTH
DT        US/M         2  SONIC TRANSIT TIME
RHOB      K/M3         3  BULK DENSITY
NPHI      V/V          4   NEUTRON POROSITY
SFLU      OHMM         5  RXO RESISTIVITY
SFLA      OHMM         6  SHALLOW RESISTIVITY
ILM       OHMM         7  MEDIUM RESISTIVITY
ILD       OHMM         8  DEEP RESISTIVITY
ILD_MSM
>>> # Whoops, typo, let's do that again
>>> las["ILD_MSM"] = 1000 / las["ILD"]
>>> print(las.curves)
Mnemonic   Unit  Value  Description
--------   ----  -----  -----------
DEPT       M            1  DEPTH
DT         US/M         2  SONIC TRANSIT TIME
RHOB       K/M3         3  BULK DENSITY
NPHI       V/V          4   NEUTRON POROSITY
SFLU       OHMM         5  RXO RESISTIVITY
SFLA       OHMM         6  SHALLOW RESISTIVITY
ILM        OHMM         7  MEDIUM RESISTIVITY
ILD        OHMM         8  DEEP RESISTIVITY
ILD_MSM:1
ILD_MSM:2

Expected behavior
This line:

>>> las["ILD_MSM"] = 1000 / las["ILD"]
>>> print(las)

Should have resulted in this:

Mnemonic  Unit  Value  Description
--------  ----  -----  -----------
DEPT      M            1  DEPTH
DT        US/M         2  SONIC TRANSIT TIME
RHOB      K/M3         3  BULK DENSITY
NPHI      V/V          4   NEUTRON POROSITY
SFLU      OHMM         5  RXO RESISTIVITY
SFLA      OHMM         6  SHALLOW RESISTIVITY
ILM       OHMM         7  MEDIUM RESISTIVITY
ILD       OHMM         8  DEEP RESISTIVITY
ILD_MSM

Not this:

Mnemonic   Unit  Value  Description
--------   ----  -----  -----------
DEPT       M            1  DEPTH
DT         US/M         2  SONIC TRANSIT TIME
RHOB       K/M3         3  BULK DENSITY
NPHI       V/V          4   NEUTRON POROSITY
SFLU       OHMM         5  RXO RESISTIVITY
SFLA       OHMM         6  SHALLOW RESISTIVITY
ILM        OHMM         7  MEDIUM RESISTIVITY
ILD        OHMM         8  DEEP RESISTIVITY
ILD_MSM:1
ILD_MSM:2

Software versions (please complete the following information):

• lasio version (import lasio; print(lasio.__version__)) master branch

Additional context
The addition of :1, :2 etc should only happen when using LASFile.read