Added Session Management

Previous version called Connect and Disconnect methods on every call. I
took this approach to simplify end user implementation. This was a bad
model based on the number of identical calls and more importantly did
not allow for multi-array session management. The later can now be
achieved as follows:

$FlashArray1 = "0.0.0.0"
$FlashArray2 = "0.0.0.1"
$MyToken = Get-PfaAPIToken -FlashArray $FlashArray1 -Username USER
-Password PWD
$MyToken1 = Get-PfaAPIToken -FlashArray $FlashArray2 -Username USER
-Password PWD

$Session = Connect-PfaController -FlashArray $FlashArray1 -APIToken
$MyToken.api_token
$Session1 = Connect-PfaController -FlashArray $FlashArray2 -APIToken
$MyToken1.api_token

Get-PfaHosts -FlashArray $FlashArray1 -Session $Session
Get-PfaHosts -FlashArray $FlashArray2 -Session $Session1

Disconnect-PfaController -FlashArray $FlashArray1 -Session $Session
Disconnect-PfaController -FlashArray $FlashArray2 -Session $Session1

The end user is now responsible for connecting and disconnecting from
the individual sessions.