SPI Clock Frequency fix, added handling GPIOs

Fixed maximum SPI clock frequency to 80 % of master clock frequency.

ACAN2517FDSettings::RequestedMode and ACAN2517FD::OperationMode types have been merged: now, use ACAN2517FDSettings::OperationMode. Consequently, the NormalFD, Sleep, InternalLoopBack, ListenOnly, Configuration, ExternalLoopBack, Normal20B and RestrictedOperation enumeration constants are available for ACAN2517FDSettings::OperationMode enumeration type.

Added the ACAN2517FD::setOperationMode method.

Added the ACAN2517FD::performSleepModeToConfigurationMode method.

Added handling of GPIO0, GPIO1 and XSTBY.

Optimized CS handling

Merged #31 and #32 pull requests from Flole998.

2.1.9

Added end method, and the LoopBackTestEndFunctionTeensy3x demo sketch.

Added CANMessage and CANFDMessage class union members

Added data_s64, data_s32, data_s16 and data_s8 to CANMessage and CANFDMessage class union members (thanks to tomtom0707).

Changed receive message internal handling

As Flole998 pointed out to me, the incoming messages were lost if the driver receive buffer was full, even though there is still room in the MCP2517FD receive buffer. In this release, messages are lost if the driver receive buffer and the MCP2517FD receive buffer are both full.

Other changes:

• Added the resetHardwareReceiveBufferOverflowCount method;
• Fixed several typos in documentation.

x9 and x10 data bit rate factors, Arduino Uno -- MCP2518FDClick wiring scheme

Added x9 and x10 data bit rate factors (thanks to Pedro Dionisio Pereira Junior).
Added Arduino Uno -- MCP2518FDClick wiring scheme in documentation (thanks to soso49)

Fixed retransmission attempts setting bug

Fixed retransmission attempts setting bug
Added NoRetransmissionAttemptsDemoTeensy3x.ino sketch.

Bug fix and improved diagInfos method (thanks to turmary)

Fix: mHardwareTxFIFOFull = true will block the transmitter if call begin() multiple times without constructor;
Add: diagInfos() could also read register BDIAG0_REGISTER.

Add method to read the diagnostic registers (thanks to Flole998).

Add method to read the diagnostic registers (thanks to Flole998).

Fix retransmission attempts settings

Fix retransmission attempts settings (thanks to Flole)