Implement user_valid_point in usrtm_valpt.c.
If custom addressing is used, this function must translate the string form of the address to its corresponding domain and offset. If the address is invalid or the address is out of the range, the point should be returned as invalid.
Regardless of addressing mode, if the device has configurable memory, this function should determine whether the point address is a valid one. One method for accomplishing this is to attempt to read the data value from the configured location in the device's memory. A second method is to read the memory configuration from the device and use this information to determine if the address is valid.