If installments is set on the contribution form it should be passed to the membership function

If installments has been set in $this->_params it should be passed through to the membership processing function. Installments would most likely be set by a hook - but it is almost impossible for an extension to get to set it via other hooks.

Background - the functionality around recurring membership has been 'bouncy' with 4.5 changing the way a separate-membership-payment-with-contribution worked and 4.6 changing it back again & imposing a bunch more rules. Hence until we have an opportunity to really re-work it we need to allow hooks considerable control as things like setting a membership payment to run for a certain number of installment/ termss are not possible in 4.6 otherwise.

Note that my thinking is that probably we should eventually move to having the recur settings on the price field & price field value since the 'normal' requests we get are to offer the same membership with different recur settings for different prices and to be able to set one or both of the contribution & membership payment to being recur. The current config is very confusing.