Rework API and header files to make dev registers more accessible to the integrating developer

[stephendpmurphy]

Currently the registers are hidden away from the developer and they do not have access to modifying individual bits as they would like.

Re-work the source along with #11 so the developer has access to everything, but can also choose to use nice APIs for the gyro, accel, and mag which configures things for you.

This means making a generic API which takes a byte buffer, length and address and writes the data blindly. This gives the developer full control of writing and reading data if they choose.

int8_t icm20948_writeRegister(uint8_t addr, uint8_t *buff, uint16_t len)