Unusual multi-FCS files

Hi there,

I've come across FCS files (From the Luminex Muse), which implement multi-FCS by simple concatenating single FCS files together. This was my solution to split them:
```
files = glob.glob("ADM_*.VIA.FCS")
for f in files:
handle = open(f,"rb")
data = handle.read()
##Some FCS files are just literal concatenations of single FCS files, this splits them.
split_data = data.split(b"FCS3.0")
for s in range(1,len(split_data)):
handle = open(f+"_"+str(s)+".FCS","wb")
handle.write(b"FCS3.0"+split_data[s])
handle.close()
```

Once these multi-FCS files are split, fcsparser works perfectly, as far as I can tell. But it might be nice for the library to be able to detect these files by default! See attached for an example FCS:
[ADM_09SEP2020_181310.VIA.FCS.zip](https://github.com/eyurtsev/fcsparser/files/5199195/ADM_09SEP2020_181310.VIA.FCS.zip)

Start by reproducing the issue with the attached ADM_09SEP2020_181310.VIA.FCS.zip example and the existing fcsparser parsing entry point. Determine how concatenated FCS files should be detected and exposed, then add coverage if the repository has relevant parser tests; done means the example can be handled without manual splitting while ordinary FCS files still parse correctly.

Written by the indexing model from the issue text.