Add a way to remap I2C1 pins after construction of standard Wire instance

[gmcn42]

Normally there is no way to edit the dev_flags variable after Wire has been constructed. It is sometimes helpful to do so though. Example: u8g2 uses the builtin Wire instance to communicate with I2C displays which is normally set to the standard pins.

Using the added functions from this PR it is possible to:

//during setup():
uint8_t df = Wire.getDevFlags();
df |= I2C_REMAP;
Wire.setDevFlags(df);

After that u8g2 will nicely communicate with an I2C display connected to the alternate pins.

[stevstrong2]

@gmcn42 , could you eventually define these functions in the header file only, so that the Wire.cpp remains untouched?

[gmcn42]

Sure. Amended PR.

[stevstrong]

I think there is a small mistake, the input parameter should be df if I'm not wrong. Can you please correct that?