More flexibility for sessionContext handling

[OGKevin]

Steps to reproduce:

1. Try and find out if your session context is expired

What should happen:

1. There is a way to know if your session has expired without resetting/calling ensureSesiosActive()

What happens:

1. There is no way to achieve such thing

Extra info:

• Tested on https://github.com/bunq/sdk_csharp/tree/0.12.0

Why

• This way you wont have to execute this code before each call but only if your session did expire

apiContext.EnsureSessionActive();
apiContext.Save(FILENAME_CONTEXT_CONF);

Which means you will be saving an unchanged context on each call

[OGKevin]

closing due to #33