Fix BCF field-name mismatches between R and Python JSON schemas

[andrewherren]

Part of #315 (cross-version deserialization, RFC 0002). Prerequisite for future cross-platform (R↔Python) serialization.

Three BCF field names differ between R and Python. Standardize R to match Python, and add legacy-name aliases on read (with a deprecation warning) so that JSONs serialized before this fix can still be loaded.

Field	Python (canonical)	R (current)	Action
BCF initial variance	sigma2_init	initial_sigma2	Rename in R write; accept old name on read
BCF adaptive coding b0	b0_samples	b_0_samples	Rename in R write; accept old name on read
BCF adaptive coding b1	b1_samples	b_1_samples	Rename in R write; accept old name on read

Note: the R in-memory object names ($b_0_samples, $b_1_samples) are unchanged — this is a JSON field name change only.

Affects: R/bcf.R