Add support for chained fit files

From the SDK:
> The FIT protocol allows for multiple FIT files to be chained together
> in a single FIT file. Each FIT file in the chain must be a properly
> formatted FIT file (header, data records, CRC).

The implementation is to note the total filesize when first loading the
file, then check if there's more data after we finish processing the
number of bytes the initial header specifies. If there is more data, try
to parse it as a fit header, then continue as normal.

fitparse/base.py

@@ -1,4 +1,5 @@
 import io
+import os
 import struct
 
 from fitparse.processors import FitFileDataProcessor
@@ -25,17 +26,14 @@ def __init__(self, fileish, check_crc=True, data_processor=None):
             self._file = open(fileish, 'rb')
 
         self.check_crc = check_crc
-
-        self._accumulators = {}
-        self._bytes_left = -1  # Not valid until after _parse_file_header()
-        self._complete = False
-        self._compressed_ts_accumulator = 0
-        self._crc = 0
-        self._local_mesgs = {}
-        self._messages = []
         self._processor = data_processor or FitFileDataProcessor()
 
-        # Start off by parsing the file header (makes self._bytes_left valid)
+        # Get total filesize
+        self._file.seek(0, os.SEEK_END)
+        self._filesize = self._file.tell()
+        self._file.seek(0, os.SEEK_SET)
+
+        # Start off by parsing the file header (sets initial attribute values)
         self._parse_file_header()
 
     def __del__(self):
@@ -89,6 +87,16 @@ def _read_and_assert_crc(self, allow_zero=False):
     # Private Data Parsing Methods
 
     def _parse_file_header(self):
+
+        # Initialize data
+        self._accumulators = {}
+        self._bytes_left = -1
+        self._complete = False
+        self._compressed_ts_accumulator = 0
+        self._crc = 0
+        self._local_mesgs = {}
+        self._messages = []
+
         header_data = self._read(12)
         if header_data[8:12] != b'.FIT':
             raise FitParseError("Invalid .FIT File Header")
@@ -122,10 +130,15 @@ def _parse_message(self):
         if self._bytes_left <= 0:
             if not self._complete:
                 self._read_and_assert_crc()
+
+            if self._file.tell() >= self._filesize:
                 self._complete = True
                 self.close()
+                return None
 
-            return None
+            # Still have data left in the file - assuming chained fit files
+            self._parse_file_header()
+            return self._parse_message()
 
         header = self._parse_message_header()