Allow assign_field() to descend into a simple (non-repeating) field (#55)

[CedricConday]

Addresses item 1 of #55

msg = hl7.parse('MSH|^~\\&|field|rep1~rep2|')
seg = msg.segments('MSH')[0]
seg.assign_field('NewRep', 3, 1)
# TypeError: 'str' object does not support item assignment

Cause

A field parsed as a single value is stored as a bare str (e.g. field 3 is Field(['field'])), whereas a repeating field holds Repetition objects. When assign_field() addresses a repetition (or component/subcomponent) of a simple field, field(repeat_num) returns that bare string and the subsequent repetition[:] = [value] slice-assignment raises.

Fix

Before descending, promote a bare string to a Repetition (and, one level down, a single-component repetition to a Component), preserving the existing value — which is the behaviour the issue asks for. Assigning the whole field/repetition still replaces it as before.

Tests

Added test_assign_deeper_into_simple_field covering the reported repetition case plus the component and subcomponent levels:

• assign_field('NewRep', 3, 1) → field 3 = NewRep
• assign_field('X', 3, 1, 2) → field 3 = field^X
• assign_field('Y', 4, 1, 1, 2) → field 4 = rep1&Y~rep2

These fail on main (TypeError) and pass with the fix. Full suite stays green (101 tests); the existing test_assign (assignment into empty fields) is unaffected.