Add optional word_len parameter to read_area/write_area

[gijzelaerr]

Summary

Fixes #559

write_area() and read_area() previously hardcoded WordLen.Byte for all non-timer/counter areas, making it impossible to perform bit-level operations through the standard API.

Root cause analysis

The S7 protocol supports multiple word lengths for area access (S7WLBit, S7WLByte, S7WLWord, etc.), and the original Snap7 C library exposes this parameter in Cli_WriteArea() / Cli_ReadArea(). However, the python-snap7 wrapper hardcoded the word length selection based solely on the area type:

if area == Area.TM:
    word_len = S7WordLen.TIMER
elif area == Area.CT:
    word_len = S7WordLen.COUNTER
else:
    word_len = S7WordLen.BYTE  # <-- always Byte, no way to override

This meant users could not write individual bits using WordLen.Bit with the byte_index*8 + bit_index addressing scheme described in the Snap7 documentation.

Fix

Added an optional word_len: Optional[WordLen] = None parameter to both read_area() and write_area():

• When word_len is None (the default), the existing area-based logic is preserved — fully backwards compatible
• When word_len is provided, it is used directly after mapping from the public WordLen enum to the internal S7WordLen enum (they share identical integer values)

This enables bit-level writes like:

client.write_area(Area.DB, 1, byte_index * 8 + bit_index, data, word_len=WordLen.Bit)

Test plan

• Existing tests pass (326 passed)
• Verify bit-level write with a real PLC using WordLen.Bit
• Verify backwards compatibility — calls without word_len parameter behave identically

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