Update file existence checks in goostats.sh

The `-a` option for `if` does not work in all environments. I have tested the script below in three different environments, Git Bash, MobaXTerm and RedHat Linux and they all produce the following output:

# The Script
```
#!/bin/bash
touch checkForfile.md
echo WITH -a
if [ -a checkForfile.md ]; then echo yes; else echo no;fi
if [ -a NocheckForfile.md ]; then echo yes; else echo no;fi
echo WITH ! -a
if [ ! -a checkForfile.md ]; then echo no; else echo yes;fi
if [ ! -a NocheckForfile.md ]; then echo no; else echo yes;fi
echo WITH -e
if [ -e checkForfile.md ]; then echo yes; else echo no;fi
if [ -e NocheckForfile.md ]; then echo yes; else echo no;fi
echo WITH ! -e
if [ ! -e checkForfile.md ]; then echo no; else echo yes;fi
if [ ! -e NocheckForfile.md ]; then echo no; else echo yes;fi
echo WITH -f
if [ -f checkForfile.md ]; then echo yes; else echo no;fi
if [ -f NocheckForfile.md ]; then echo yes; else echo no;fi
echo WITH ! -f
if [ ! -f checkForfile.md ]; then echo no; else echo yes;fi
if [ ! -f NocheckForfile.md ]; then echo no; else echo yes;fi
```

# The output
```
 $ ./checkforfile.sh
WITH -a
yes
no
WITH ! -a
no
no
WITH -e
yes
no
WITH ! -e
yes
no
WITH -f
yes
no
WITH ! -f
yes
no
```

# Expected output
The expected output for all tests would be:
```
yes
no
```
However, the `-a` option does not work if used in conjunction with not ('!')

Author: jsteyn

shell-lesson-data/north-pacific-gyre/goostats.sh

@@ -11,11 +11,11 @@ then
 fi
 
 # check if files already exist (good for $1, bad for $2)
-if [ ! -a "$1" ]
+if [ ! -f "$1" ]
 then
     echo "error reading input: $1"
     exit 2
-elif [ -a "$2" ]
+elif [ -f "$2" ]
 then
     echo "error writing result: $2"
     exit 2